COVID-19 Vaccine Studies

COVID-19 Vaccine Research Studies

To date 5 COVID-19 vaccines have been authorised for use in Europe by the EMA.

Since the COVID-19 vaccines have been used in Ireland and worldwide, research has been conducted all over the world that demonstrates the safety and effectiveness of COVID-19 vaccines against the COVID-19 virus.

Although there may be recommendations made in external reports - they are for information only. 

Please note that the HSE clinical guidance on COVID-19 vaccines should be referred to for up to date guidance for vaccinators in Ireland.

Read Comirnaty 5 to 11 year olds Clinical Trial and Real World Data


31 March 2022

Immediate side effects of Comirnaty COVID-19 vaccine: A nationwide survey of vaccinated people in Israel, December 2020 to March 2021

A large-scale study from Israel reviewed short term systemic events within 3 days after receiving the Pfizer Comirnaty vaccine. Data was gathered directly from the vaccine recipients.

The prevalence of systemic side effects (fatigue, headache and myalgia) after the Comirnaty vaccine was lower among the survey respondents than reported in the clinical trials but higher than the cases reported by passive surveillance to the Israeli Ministry of Health. This discrepancy may indicate an under-reporting pattern of mild symptoms that do not require medical follow-up and do not interfere with daily routines.

After controlling for multiple factors, including diabetes, immunosuppression, cancer, obesity) and pregnancy status, adverse effects were more frequently reported in younger individuals and women, but less frequently among pregnant women.

These findings present valuable information for individuals considering vaccination and for healthcare professionals. It is particularly important in this group of patients to emphasize that the study showed the side effects are mild after Comirnaty vaccine and therefore not an indication to avoid vaccination.


15 February 2022

Effectiveness of Maternal Vaccination with mRNA COVID-19 Vaccine During Pregnancy Against COVID-19–Associated Hospitalization in Infants Aged 6 Months

Effectiveness of maternal completion of a 2-dose primary mRNA COVID-19 vaccination series during pregnancy against COVID-19 hospitalization among infants aged <6 months was 61% (95% CI = 31% to 78%).

Effectiveness of completion of the primary COVID-19 vaccine series early and later in pregnancy was 32% (95% CI = –43% to 68%) and 80% (95% CI = 55% to 91%), respectively.

Completion of a 2-dose mRNA COVID-19 vaccination series during pregnancy might help prevent COVID-19 hospitalization among infants aged <6 months.

27 January 2022

Extended interval BNT162b2 (Pfizer) vaccination enhances peak antibody generation in older people

This was a cohort study in the UK, including 175 participants aged 80 and older. 99 participants received two doses 3 weeks apart (standard interval). 73 participants received the two doses 11-12 weeks apart (extended interval). 15 participants were excluded as serology indicated they had previous SARS-CoV-2 infection. Antibody responses were compared in both groups. Spike-specific antibodies were detected in 100% of participants in both groups 2-3 weeks after the second dose.The magnitude of the antibody response was compared in the two groups. Antibody titres in the standard-interval regimen peaked at 1138 U/ml after the second dose and then fell by 2.6-fold over the subsequent weeks (p<0.0001). Within the extended-interval cohort the median antibody titre was 17 U/ml at 5-6 weeks after the first vaccine but showed a substantial 242-fold increase to reach 4030 after the second boost (p<0.0001). A comparison of the median magnitude of peak cellular responses after the second vaccine in the two schedules showed that these were higher for donors in the standard-interval regime (72 vs 20 spots/million; p<0.0001).

Parry H, et al. Vaccines;

27 November 2021

Combining Influenza and COVID-19 Vaccination (ComFluCOV) study

Concomitant vaccination with an m RNA  vaccine  plus an age-appropriate influenza vaccine raises no safety concerns and preserves antibody responses to both vaccines. Concomitant vaccination with both COVID-19 and influenza vaccines over the next immunisation season should reduce the burden on health-care services for vaccine delivery, allowing for timely vaccine administration and protection from COVID-19 and influenza for those in need.

Read more here:

2 November 2021 

Real-world effectiveness of Ad26.COV2.S adenoviral vector vaccine for COVID-19

This study used surveillance data in the USA comparing those who had received one dose of Janssen vaccine to unvaccinated individuals, comparing outcomes. It estimates a vaccine effectiveness of 76.7% in preventing SARS-CoV-2 infection with onset at least two weeks after vaccination. Due to this vaccine being introduced relatively recently, data was not available to robustly study the effect of the Janssen vaccine on outcomes of mortality, hospitalisation and ICU admissions. This study provides real world data on the effectiveness of Janssen vaccine in preventing COVID-19 infection.

Corchado-Garcia J, et al.

28 October 2021

Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households

This study evaluated data from 194,362 household members (which represented 92,470 households of 2 to 14 persons per household) of 144,525 health care workers who had been employed during the period from March 2020 through November 2020. The study compared cases and hospitalisations due to Covid-19 in household members of both vaccinated and unvaccinated health care workers.

The rate of infection with COVID-19 for people that live with healthcare workers was found to be at least 30% lower when the worker has been vaccinated with a single dose. Where healthcare workers had received a second dose of the vaccine at least 14 days before, their household members had a rate of COVID-19 which was at least 54% lower than household members where healthcare workers had not been vaccinated.

Anoop SVS et al.


 14 October 2021

Receipt of mRNA Covid-19 Vaccines in pregnancy: analysis from the Centers for Disease Control and Prevention (CDC) in the US

This analysis from the from the Centers for Disease Control and Prevention (CDC) in the US, reported the rates of miscarriage in women who had received at least one dose of an mRNA COVID-19 vaccine either before conception (30 days before the first day of the last menstrual period through 14 days after) or in pregnancy before 20 weeks of gestation.

They compared rates of miscarriage between 6 and 20 weeks of gestation in women who had received the vaccine, compared with rates in the general population.

The analysis found no difference between women vaccinated with an mRNA vaccine and the rates in the general population. These findings add to the accumulating evidence about the safety of mRNA Covid-19 vaccination in pregnancy.

Read more here

Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK

This large community-based survey of randomly selected households across the UK examined vaccine effectiveness against the COVID-19 Delta variant in those aged 18 years and older. This study found that the effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines is reduced with the Delta variant. Two doses of the Pfizer-BioNTech or Oxford-AstraZeneca vaccine provided the same protection as having previous infection. Those who were vaccinated after infection had more protection than those vaccinated without having had prior infection. Protection was higher among younger age groups and the time between vaccine doses did not affect effectiveness. Compared to Oxford-AstraZeneca vaccines, the Pfizer-BioNTech vaccine has greater initial effectiveness but this declines faster than Oxford-AstraZeneca. A single dose of the Moderna vaccine had similar or greater effectiveness compared to a single dose of the Pfizer-BioNTech or Oxford-AstraZeneca vaccines. For those who were infected with the Delta variant after vaccination, peak levels of virus were similar to those in unvaccinated individuals. Overall, this study found that obtaining two vaccine doses is the most effective way to ensure protection against the COVID-19 Delta variant, however vaccine effectiveness is reduced and peak viral load is higher with the Delta variant.

Read more here

29 August 2021

COVID-19-associated hospitalizations among vaccinated and unvaccinated adults ≥18 years

This paper, available in pre-print and not yet peer-reviewed, compared hospitalisation rates from COVID-19 in the U.S. in vaccinated and unvaccinated individuals.

Population-based hospitalization rates show that unvaccinated adults aged ≥18 years are 17 times more likely to be hospitalized compared with vaccinated adults.

Rates are far higher in unvaccinated persons in all adult age groups, including during a period when the Delta variant was the predominant strain of the SARS-CoV-2 virus.

FP Havers, et al

medRxiv 2021; DOI:

27 July 2021 

Risk of Myocarditis from COVID-19 Infection in People Under Age 20: A Population-Based Analysis (Pre-print)

This US study examined medical records from 48 healthcare organisations and included patients aged less than 20 years who had a COVID-19 diagnosis between April 2020 and March 2021. The primary outcome was a diagnosis of myocarditis within 90 days of a diagnosis of COVID-19 or a positive COVID-19 test. In total 14,207 patients aged 12-17 years were included in the study. This study found that for males aged 12-17 years, the adjusted rate of myocarditis was 450 per million cases of COVID-19. For females aged 12-17 years, the adjusted rate of myocarditis was 213 cases per million cases of COVID-19. Myocarditis occurred within 5 days in 40% or from 19-82 days in 60%. Two patients were hospitalised and there were no deaths reported. There is a risk of myocarditis associated with receiving an mRNA COVID-19 vaccine. This risk is highest in those aged 12-17 years and is higher in males compared to females. Overall, this study concluded that young males aged 12-17 years infected with the virus are up to 6 times more likely to develop myocarditis as those who receive the vaccine and that young females aged 12-17 years infected with the virus are up to 21 times more likely to develop myocarditis as those who receive the vaccine.

Read more here

21 July 2021

UK Data on the Effectiveness of the Pfizer-BioNTech and AstraZeneca COVID-19 Vaccines Against the B.1.617.2 variant

This study looked at the effectiveness of COVID-19 vaccines against the B.1.617.2 variant.

They found a reduction in effectiveness of one dose of vaccine against symptomatic disease with the B.1.617.2 variant.

Vaccine effectiveness against symptomatic disease with B.1.617.2 for a single dose of either vaccine was approximately 33%, for two doses of Pfizer vaccine effectiveness was approximately 88% and for two doses of AstraZeneca is approximately 60%.

This study estimates that there is reduced effectiveness of COVID-19 vaccines after one dose of Pfizer or AstraZeneca vaccine, however two doses of vaccine appeared to provide significant protection. Of note there was a limited follow up period in this study so they were unable to estimate effectiveness against severe illness, hospitalisation and death.

Read more here:

10 July 2021

Immunogenicity and reactogenicity of BNT162b2 booster in ChAdOx1-S-primed participants (CombiVacS): a multicentre, open-label, randomised, controlled, phase 2 trial

They performed a phase 2, open-label, randomised, controlled trial on adults aged 18–60 years, vaccinated with a single dose of Vaxzevria® vaccine 8–12 weeks before screening, and no history of previous SARS-CoV-2 infection.

Participants were randomly assigned (2:1) to receive either Comirnaty® COVID-19 Vaccine (0·3 mL) via a single intramuscular injection (intervention group) or continue observation (control group). The control group did not routinely receive a second dose of Vaxzevria®, but it could be used if requested by the participant or established by local health authorities.

The primary outcome was 14-day immunogenicity, measured by immunoassays for SARS-CoV-2 trimeric spike protein and receptor binding domain (RBD). The safety outcome was 7-day reactogenicity, measured as solicited local and systemic adverse events.

663 (98%) participants (n=441 intervention, n=222 control) completed the study up to day 14. 

In the intervention group, the mean antibody levels increased from 71·46 BAU/mL. 

Reactions were mild (n=1210 [68%]) or moderate (n=530 [30%]), with injection site pain (n=395 [88%]), induration (n=159 [35%]), headache (n=199 [44%]), and myalgia (n=194 [43%]) the most commonly reported adverse events. No serious adverse events were reported.

Comirnaty® given as a second dose in individuals prime vaccinated with Vaxzevria® induced a robust immune response, with an acceptable and manageable reactogenicity profile.

Borobia AM A et al

The Lancet. VOLUME 398, ISSUE 10295, P121-130. ; DOI:

25 June 2021

Safety and Immunogenicity Report from the Com-COV Study – a Single-Blind Randomised Non-Inferiority Trial Comparing Heterologous And Homologous Prime-Boost Schedules with An Adenoviral Vectored and mRNA COVID-19 Vaccine

This study examined whether a mixed vaccine schedule was at least as good as a standard vaccine schedule at producing higher levels of SARS CoV 2 antibodies 28 days after the booster vaccine dose.

463 people who had never received a COVID-19 vaccine previously aged 50 years and over, with no or well-controlled mild-moderate comorbidities.

The findings demonstrate that all the schedules studied (ChAd/BNT, BNT/BNT or BNT/ChAd) induced concentrations of antibodies to SARS CoV 2 at least as high as those induced after a licensed ChAd/ChAd schedule, which is effective in preventing symptomatic COVID-19 when administered at a 4-12 week primeboost interval.

Administering a booster vaccine of either the same or alternative vaccines 28 days after the prime vaccination produced higher levels of SARS CoV 2 antibodies than a single dose of either vaccine alone.

Liu X, et al. The Lancet DOI:

23 June 2021

Threat Assessment Brief: Implications for the EU/EEA on the spread of the SARS-CoV-2 Delta (B.1.617.2) variant of concern

This report from the European Centre for Disease Prevention and Control (ECDC) highlights the latest evidence on the Delta variant of concern including:

  • The Delta variant is likely 40-60% more transmissible than the Alpha variant (and is likely associated with an increased risk of hospitalisation)
  • Based on modelling data they expect 90% of COVID-19 cases in Europe to be due to the Delta variant by the end of August. They also model the impact of relaxing non-pharmaceutical interventions (such as social distancing , face masks and respiratory/ hand hygiene measures) on case rates, hospitalisations and deaths.

The report also highlights the importance (for both the general population and those at high risk of severe-COVID-19) of completing the full vaccination schedule (for a two dose COVID-19 vaccine regime) to get the best protection against COVID-19 in particular for the Delta variant.

Read more here

14 June 2021 

Importance of the second dose in preventing hospitalisation from the Delta variant

The latest analysis (published as a pre-print) from Public Health England provides further evidence that the vaccines are highly effective against preventing hospitalisation from the Delta variant (B.1.617.2) in England after 2 doses: Comirnaty (Pfizer BioNTech) is 96% effective and Vaxzevria (AstraZeneca) is 92% effective. The vaccines have a similar effectiveness against the Alpha variant (B.1.1.7). Further analysis is underway to understand the impact of vaccination on deaths due to the Delta variant (it is expected to offer a high level of protection similar to other variants).

Public Health England, Press Release. Read more here

SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness

This correspondence published in the Lancet compares the demographics, risk of hospitalisation and vaccine effectiveness for the Delta (B.1.617.2) variant when compared with the Alpha (B.1.1.7) variant.

The Delta variant was found to be prevalent across all ages but particularly in children aged 5-9 years and higher socioeconomic groups. The Delta variant was also associated with a significantly increased risk of hospitalisation (nearly twice that compared to the Alpha variant) particularly in those with multiple comorbidities.

Vaccination (two weeks after the second dose) was effective at reducing hospitalisation:

  • Comirnaty (Pfizer/BioNTech) was 92% effective against hospitalisation with the Alpha variant and 79% effective against hospitalisation with the Delta variant.
  • Vaxzevria (AstraZeneca) was 73% effective against hospitalisation with the Alpha variant and 60% effective against hospitalisation with the Delta variant.

Sheikh A, et al

The Lancet ; DOI:

10 June 2021 

Community-level evidence for SARS-CoV-2 vaccine protection of unvaccinated individuals

Publication from Israel studies the impact on infection rates in children under the age of 16 (who are unvaccinated) from vaccinating adults with the Pfizer-BioNTech COVID-19 vaccine.

They studied the impact of vaccination on 177 communities with low underlying natural immunity.

They found a correlation between high vaccination rates and lower infection rates later in unvaccinated children within the same community. They estimate that by vaccinating an additional fifth of the population in a community (aged 16-50) this results in nearly a two-fold reduction in positive test fraction within the unvaccinated cohort (children under 16 years of age).

Milman O, et al

Nat Med 2021, DOI;

9 June 2021 

Impact of vaccination on SARS-CoV-2 cases in the community: a population-based study using the UK’s COVID-19 Infection Survey

This study examined the impact of vaccination on laboratory confirmed COVID-19 infection rates. Data included swab results from over 370,000 people between December 2020 and April 2021 A 65% reduction in infections was noted following a single dose of both vaccines and 70% reduction following two doses of Comirnaty® (second doses of Vaxzevria® were not widely rolled out yet). Furthermore, infections in those who had been vaccinated had a lower viral load; therefore less likely to transmit the virus. The vaccines appear to be effective against the B.1.1.7 variant that is dominant in the UK and in Ireland.

Pritchard E et al.

27 May 2021

Post-vaccination SARS-CoV-2 infection: risk factors and illness profile in a prospective, observational community-based case-control study

A study, published in The Lancet Infectious Diseases, found that for those who had completed a two course Covid-19 vaccine schedule but went on to develop a Covid-19 infection had a significantly lower risk of developing “long COVID”. Their risk was reduced by almost half (49%).

Hospitalisations were 73% less likely and there were less acute symptoms (31% less) among those fully vaccinated. 

These findings are the first to suggest that although there is evidence of effectiveness for the reduction in Covid-19 infection in those who are fully vaccinated, the risk of long Covid post vaccination infection is also reduced by completing the Covid-19 vaccination schedule.

M Antonelli, et al

medRxiv 2021; DOI:

Pfizer-BioNTech COVID-19 clinical trial finds effectiveness in children aged 12-15

This phase 3 clinical trial involved over 2200 children aged 12-15 who received two doses of the Pfizer-BioNTech COVID-19 vaccine 3 weeks apart. They compared the efficacy, safety and the immune response when compared to those aged 16-25.

Most of the side effects reported were mild to moderate and short lived (similar to those seen in older age cohorts). Furthermore laboratory tests confirmed that children aged 12-15 mounted a greater immune response when compared to young adults aged 16-25. 100% (95% CI, 75.3 to 100) efficacy is noted against symptomatic disease in children aged 12-15 after 7 days after the second dose.

Read more here:

13 May 2021

Effectiveness of BNT162b2 mRNA and ChAdOx1 adenovirus vector COVID-19 vaccines on risk of hospitalisation among older adults in England: an observational study using surveillance data

This study looked at the risk of hospitalisation, comparing those who are vaccinated and unvaccinated in England, using surveillance data for the first four months of the vaccination programme. In those aged 80 years and over, vaccine efficacy against hospitalisation was 73% following the first dose of AstraZeneca vaccine and 81% following the first dose of the Pfizer vaccine, and 93% following the second dose of the Pfizer vaccine. In those aged 70-79 years, vaccine effectiveness against hospitalisation was 84% following the first dose of Astrazeneca vaccine and 81% following the first dose of the Pfizer vaccine. This large observational study provides further real world effectiveness data for the Pfizer and Astrazeneca vaccines.

Ismail SA, et al

7 May 2021 

Effectiveness of Pfizer-BioNTech and Moderna Vaccines Against COVID-19 Among Hospitalized Adults Aged ≥65 Years — United States, January–March 2021

This case control study examines the effectiveness of the mRNA based COVID-19 vaccines (Comirnaty® from Pfizer/ BioNTech and COVID-19 vaccine Moderna®) against hospitalisation in older adults (aged 65 years and older) across various hospital sites in the United States between January and March 2021. The sample size was 417 patients. They estimate that the vaccines are 94% effective at reducing hospitalisation with COVID-19 after being fully vaccinated (2 weeks after second dose) and 64% effective following partial vaccination (2 weeks after first dose). Effectiveness in a real-world setting was therefore very similar to that seen in the trials of the vaccines.

Tenforde MW et al

MMWR Morb Mortal Wkly Rep 2021; DOI

6 May 2021

Association Between Vaccination With BNT162b2 and Incidence of Symptomatic and Asymptomatic SARS-CoV-2 Infections Among Health Care Workers

This retrospective single site cohort study in Israel estimated the effect of Pfizer vaccine on SARS-CoV-2 infection in healthcare workers (symptomatic and asymptomatic). They estimated vaccine efficacy of 97% for preventing symptomatic infection and 86% for asymptomatic infection. This study provides further evidence that the vaccine is highly effective in preventing SARS-CoV-2 infection.

Angel Y, et al

5 May 2021

Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data

This observational study reviews the impact of the national roll out of the Comirnaty® (PfizerBioNTech) COVID-19 mRNA Vaccine on outcomes in Israel.

By the start of April over 70% of the adult population (aged 16 and over) were fully vaccinated (21 days interval between doses). 7 days after completing the full vaccination schedule the following outcomes were found: 95% effectiveness against COVID-19 and 92% against asymptomatic COVID-19. It was around 97% effective against symptomatic COVID-19, COVID-19 related hospitalisation and deaths. The vaccine was effective against the B.1.1.7 (Kent) variant. This study provides further large-scale real-world effectiveness of this COVID-19 vaccine in line with the results from clinical trials.

Haas EJ et al. The Lancet:

30 April 2021 

Prior SARS-CoV-2 infection rescues B and T cell responses to variants after first vaccine dose

The immune response (antibody, T and B cell) against COVID-19 variant (B.1.1.7 and B.1.351) in healthcare workers in the UK was measured followed one dose of the Comirnaty® (PfizerBioNTech) COVID-19 mRNA Vaccine (comparing those with or without previous COVID-19 infection).

The study found that pervious infection in conjunction with a single dose of the vaccine led to a better immune response (neutralising antibodies) against variants when compared to those who were infection naive and received a single dose of the vaccine. The study highlights the importance of completing the second dose of the vaccine schedule in those who are infection naïve.

Reynolds CJ et al

21 April 2021

Preliminary Findings of mRNA Covid-19 Vaccine Safety in Pregnant Persons

This study highlights initial findings from safety reports following vaccination with mRNA COVID-19 vaccines during pregnancy in the United States between December 2020 and February 2021. Over 800 participants had a completed pregnancy; within this group maternal and foetal outcomes in the vaccinated cohort were similar to those seen in unvaccinated individuals pre-pandemic. Preliminary findings did not show obvious safety signals among pregnant persons who received mRNA Covid-19 vaccines. However, more longitudinal follow-up, including follow-up of large numbers of women vaccinated earlier in pregnancy, is necessary to inform maternal, pregnancy, and infant outcomes.Shimabukuro TT, et al

N Engl J Med 2021;

15 April 2021 

BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting

The study uses data from Israel’s largest health organisation to assess the real world effectiveness of Comirnaty® (Pfizer/BioNTech vaccine). The vaccine was given as two doses 21 days apart. A matched cohort study was undertaken involving nearly 1.2 million people (half of whom were vaccinated). Outcomes were measured after the first dose (day 14-20) and second dose (on or after day 7). This study provides real-world evidence of effectiveness of Comirnaty® vaccine against a number of clinical outcomes as summarised in Table 1. Subgroup analysis showed similar effectiveness across the ages. However, there may be marginally lower effectiveness in those with underlying health conditions. During the study period the incidence of COVID-19 was high and the dominant strain was the highly transmissible B.1.1.7 variant which was the dominant strain in Ireland.

Dagan N et al

N Engl J Med;

2 April 2021 

Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers — Eight U.S. Locations, December 2020–March 2021

This study looked at the effectiveness of Comirnaty® Pfizer/BioNTech and COVID-19 vaccine Moderna® in preventing SARS-CoV-2 infections among 3,950 healthcare workers and other essential workers over a 13-week period from December 14, 2020 to March 13, 2021.

Participants self-collected nasal swabs each week regardless of whether they had developed symptoms of illness. Researchers were able to look for evidence of SARS-CoV-2 infection irrespective of symptoms. A small number (11%) of infections in this study were asymptomatic. The majority of infections (58%) occurred among people whose infections were identified by testing before they developed symptoms or knew they were infected.

Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80% two or more weeks after vaccination with a single dose. Results showed that following the second dose of vaccine, the risk of infection was reduced by 90% two or more weeks after vaccination.

The study demonstrates that these two mRNA vaccines can reduce the risk of all SARS-CoV-2 infections, not just symptomatic infections.

Thompson MG

MMWR Morb Mortal Wkly Rep 2021; DOI:

1 April 2021

Delayed Large Local Reactions to mRNA-1273 Vaccine against SARS-CoV-2

Blumenthal et al reported on local reactions in 12 patients following administration of mRNA-1273 (COVID-19 vaccine Moderna®) vaccine.

  • Median onset of reactions occurred day 8 after the vaccine (range 4-11 days).
  • Five of the reactions were grade 3 plaques (≥10 cm in diameter)
  • Some patients had concurrent systemic adverse effects, and among these patients, 2 had additional skin findings.
  • Most patients received treatment for their symptoms (e.g., with ice and antihistamines). Some patients received glucocorticoids (topical, oral, or both), and 1 patient received antibiotic therapy for presumptive cellulitis.
  • The symptoms resolved a median of 6 days after onset (range, 2 to 11).  

Given that neither local injection-site reactions nor delayed-type hypersensitivity reactions are contraindications to subsequent vaccination, all 12 patients were encouraged to receive the second dose and completed their mRNA-1273 vaccination course.

  • Half the patients did not have a recurrence of large local reactions
  • Three patients had recurrent reactions that were similar to those after the initial dose
  • Three patients had recurrent reactions that were of a lower grade than those after the initial dose.
  • The median onset of cutaneous symptoms after the second dose (day 2; range, 1 to 3) was earlier than that after the first dose  

Given the up-scaling of COVID-19 vaccination programmes worldwide the presentation of delayed local reactions such as those described here will become more common. It is important that these adverse events  are always reported to the appropriate regulatory authorities (Health Product Regulatory Authority (HPRA) in Ireland).

Blumenthal et al

N Engl J Med 2021; DOI:

11 March 2021

Vaccine administration at a higher location than recommended and supraclavicular lymphadenopathy

This case series published in Eurosurveillance, highlights the importance of correct intramuscular injection technique and the administration of the vaccine in the correct site.

The authors report on 20 cases of acute onset of single supraclavicular lymphadenopathy coinciding with administration of a dose of an mRNA vaccine on the same side. While axillary lymphadenopathy had been reported as a recognised adverse reaction following mRNA vaccines, supraclavicular lymphadenopathy had not, and therefore this finding prompted concern amongst recipients and healthcare professionals about other possible cause, including malignancy.

Twelve of the 20 patients in the case series spontaneously reported that the intramuscular injection point was unusually high, and nearly all (17/20) acknowledged a similar perception when they were asked about this specifically (either compared with the previous dose administration or in relation to their theoretical expectation about the exact location of the point of injection).

The authors’ report that if the vaccine was given at a higher location than recommended, the supraclavicular lymph nodes and not the axillary nodes is the most frequent drainage area and therefore this may have resulted in supraclavicular lymphadenopathy.

Land marking the injection site is a crucial step to ensure administration of the vaccine at the correct site of the Deltoid muscle. This has been highlighted in this bulletin, and through training and guidance. See for further details.

Read more here

3 March 2021

Assessing the Effectiveness of BNT162b2 and ChAdOx1nCoV-19 COVID-19 Vaccination in Prevention of Hospitalisations in Elderly and Frail Adults: A Single Centre Test Negative Case-Control Study

More real world data is available from the UK, the AvonCAP study showing substantial reductions in the risk of COVID-19-related hospitalisations after the Comirnaty® (Pfizer/BioNTech COVID-19 mRNA Vaccine) and Vaxzevria® (COVID-19 Vaccine AstraZeneca) vaccines among people aged ≥ 80 and older, including frail elderly people with extensive comorbid disease.

Vaccine effectiveness against COVID-19 related hospitalisation following one dose of Comirnaty® (Pfizer/BioNTech COVID-19 mRNA Vaccine) in this population was 71.4% and 80.4% for Vaxzevria® (COVID-19 Vaccine AstraZeneca) when measured ≥14 days after the first dose.

Hyams C et al

The Lancet ; DOI:

19 February 2021

Single – dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCov-10(AZD1222) vaccine: a pooled analysis of four randomised trials

A single standard dose Vaxzevria® (COVID-19 Vaccine AstraZeneca) provided protection against primary symptomatic COVID-19 in the first 90 days with an efficacy of 76·0% (95% CI 59·3 to 85·9), there was no evidence of waning of protection in the first three months after vaccination.

Vaccine efficacy after two standard doses was 55·1% (95% CI 33·0–69·9) with an interval of less than 6 weeks and 81·3% (60·3–91·2) when more than 12 weeks apart. Higher efficacy  was reported when the interval between the primary and booster dose are given more than 12 weeks apart.

Efficacy of a single standard dose against any NAAT (nucleic acid amplification test )-positive infection was 63·9% (46·0 to 75·9) from 22 days to 90 days, suggesting the potential for a substantial reduction in transmission, although these results are exploratory and require further investigation.

Merryn V et al

The Lancet ; DOI:

4 February 2021

Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine

This is a study examining the efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine (also known as the COVID-19 Vaccine Moderna®) was published in the New England Journal of Medicine on February 4th 2021.

Efficacy:  This means that the efficacy of the vaccine in preventing symptomatic COVID-19 disease was 94.1% (95% CI 89.3%-96.8%; P<0.001) and was consistent in both younger (18-<65 years; vaccine efficacy= 95.6%) and older patients (>65 years; vaccine efficacy= 86.4%). Thirty patients in the trial developed severe COVID-19 disease all of whom were in the placebo group. This means that the mRNA-1273 SARS-CoV-2 vaccine was 100% effective at preventing severe COVID-19 disease.

Safety: Adverse events were more commonly reported in those who received two doses of the vaccine rather than two doses of the placebo (88.6% vs. 18.8%). The most commonly reported adverse events were injection-site reactions (mainly grade 1 or 2) and lasted for a mean of 2.6 and 3.2 days respectively. Most commonly reported systemic adverse events included headache, myalgia (muscle pain), arthralgia (bone pain), chills and fatigue and were more frequently reported after the second vaccine dose.

Summary: Overall this study shows that the efficacy of the COVID-19 Vaccine Moderna® vaccine in preventing both symptomatic COVID-19 and severe COVID-19 disease is high while the presence of adverse reactions post vaccine were limited to local injection-site reactions and some expected systemic reactions. Further studies examining the long-term duration of immunity of the COVID-19 Vaccine Moderna® are required.

Baden LR et al.

N Engl J Med 2021;

This page was updated 15 April 2022