BOOSTERS EFFICACY: Six different COVID-19 boosters are safe and increase immunity when given after two doses of AstraZeneca or Pfizer-BioNTech …

… with large variations in immune responses, UK trial shows

Dec 2, 2021

Key messages

  • Randomised, phase 2 trial of COVID-19 booster vaccines finds seven increase immunity when given 10–12 weeks after two doses of Oxford-AstraZeneca, and six increase immunity following two doses of Pfizer-BioNTech.
  • However, there are large variations in antibody and cellular immune responses between vaccines.
  • UK trial involving 2,878 adults aged 30 years or older shows all seven vaccines pose no safety concerns, with fatigue, headache, and injection site pain most often reported and more common in younger people.
  • Gap between second dose and booster was shorter than between first two doses for most participants, which could lead to lower immunity boost than if longer intervals were used.
  • Study assessed immune response, rather than effectiveness in protecting against infection or severe disease — although the two are closely linked, the exact relationship is unclear
  • Substantial variation in the immune responses provoked by different boosters (as identified in this study), along with in-country availability of vaccines and what level of boost is sufficient for national disease control objectives will all help inform policy decisions on boosters.

Six different COVID-19 boosters are safe and provoke strong immune responses in people who have previously received a two-dose course of ChAdOx1-nCov19 (Oxford-AstraZeneca [ChAd]) or BNT162b2 (Pfizer-BioNTech [BNT]), according to the first randomised trial of boosters given after two doses of either vaccine, published in The Lancet.

ChAd has now been deployed in more than 180 countries and BNT in more than 145 countries. Two doses of ChAd and BNT have shown 79% and 90% protection, respectively, against hospitalisation and death after six months in several studies. However, protection against COVID-19 infection wanes over time. That has driven consideration of boosters to protect the most vulnerable, lessen pressure on health services, and mitigate economic impacts. However, little data exist on the comparative safety of COVID-19 vaccines, and the immune responses they stimulate, when given as a third dose.

The COV-BOOST study looked at safety, immune response (immunogenicity) and side-effects (reactogenicity) of seven vaccines when used as a third booster jab. The vaccines studied were ChAd, BNT, NVX-CoV2373 (Novavax [NVX]), Ad26.COV2.S (Janssen [Ad26]), Moderna [mRNA1273], VLA2001 (Valneva [VLA]), and CVnCov (Curevac [CVn]).

 “The side effect data show all seven vaccines are safe to use as 3rd doses, with acceptable levels of inflammatory side effects like injection site pain, muscle soreness, fatigue. Whilst all boosted spike protein immunogenicity after two doses of AstraZeneca, only AstraZeneca, Pfizer-BioNTech, Moderna, Novavax, Janssen and Curevac did so after two doses of Pfizer-BioNTech”, comments Professor Saul Faust, trial lead and Director of the NIHR Clinical Research Facility, University Hospital Southampton NHS Foundation Trust.

 “It’s really encouraging that a wide range of vaccines, using different technologies, show benefits as a third dose to either AstraZeneca or Pfizer-BioNTech. 

That gives confidence and flexibility in developing booster programmes here in the UK and globally, with other factors like supply chain and logistics also in play”, adds Professor Faust.

 “It’s important to note that these results relate only to these vaccines as boosters to the two primary vaccinations, and to the immune response they drive at 28 days. Further work will generate data at three months and one year after people have received their boosters, which will provide insights into their impact on long-term protection and immunological memory. We are also studying two of the vaccines in people who had a later third dose after 7–8 months although results will not be available until the new year.” [1]

 A randomised, phase 2 trial of seven booster vaccines was conducted, with the third doses given 10–12 weeks after initial two-dose courses of ChAd or BNT. The trial involved 2,878 participants in good health recruited at 18 UK sites between June 1st and June 30th 2021. Participants had received their first doses of ChAd or BNT in December 2020, January or February 2021, and second doses at least 70 days before enrolment for ChAd and at least 84 days for BNT. Around half of participants received two doses of ChAd and half two doses of BNT. The control vaccine used was a meningococcal conjugate vaccine (MenACWY). [2]

 Participants were aged 30 years or older, with approximately half aged 70 years or older. The average age of participants who received ChAd was 53 years in the younger age group and 76 years in the older age group. Average ages for BNT were 51 and 78 years, respectively.

 Thirteen experimental and control arms of the trial (seven vaccines plus three at half dose and 3 control arms) were split into three participant groups, with six sites per group. Group A received NVX, half dose NVX, ChAd, or a control. Group B received BNT, VLA, half dose VLA, Ad26 or a control. Group C received Moderna, CVn, which was withdrawn from further clinical development in October 2021, half dose BNT, or a control.

 Primary outcomes were adverse effects seven days after receiving a booster, and levels of antibodies targeting the spike protein on the surface of COVID-19 virus cells — which enables them to enter human cells — after 28 days, compared to controls. Secondary outcomes included the response of T cells — which play a key role in the immune response to viral infection, and seem important in controlling disease severity — to wild type, alpha, beta, and delta variants. Both antibody-mediated immunity and T-cell response are known to be important in vaccine effectiveness.

 Increases in anti-spike protein antibody levels after 28 days varied across the vaccines. After two doses of ChAd these ranged from 1.8 times higher to 32.3 times higher according to the booster vaccine used. After two doses of BNT the range was 1.3 times higher to 11.5 times higher. Significant T-cell responses were reported in several combinations.

 At 28 days, all booster results were similar for participants aged 30–69 years and those aged 70 years or older. The authors warn that the boost ratios should be interpreted with caution because they relate to immunogenicity rather than protection against disease, and the relationship between antibody levels at day 28 and long-term protection and immunological memory is unknown.

 Reactions to all seven vaccines were similar, with fatigue, headache, and injection site pain most often reported. These were more commonly reported by those aged 30–69. 912 of the 2,878 participants experienced a total of 1036 adverse events — effects beyond these side effects, 24 of which were severe.

 The study has several limitations. Due to pandemic timelines and the need to generate data to inform policy in September 2021, the interval between second and third doses was shorter in some participants than between their first two doses. Several studies have shown that a longer time period between the first and second doses may improve immunogenicity, including improved antibody responses when the initial BNT doses are spaced by 12 weeks rather than 3 weeks. This could mean that the boost in immunity is lower than if longer dose intervals had been used. This is being investigated in a trial amendment in which third vaccine doses are offered to people who previously received a control. Only recruiting people over 30 years old limits the generalisability of the findings to younger age groups, as studies have generally shown that the vaccines tend to provoke a stronger immune response in younger people, and slightly higher rates of adverse effects. Participants were also mostly white. Not all vaccines could be randomised together due to trial design, limiting comparison between site groups, and logistical issues meant not all vaccines could be tested at half dose. Further analysis will compare vaccines in different groups.


This study was funded by the UK Vaccine Taskforce and National Institute for Health Research. It was conducted by researchers from University Hospital Southampton NHS Foundation Trust, University of Southampton, Imperial College London, University of Oxford, NIHR Oxford Biomedical Research Centre, Oxford Vaccine Group, Stockport NHS Foundation Trust, NIHR Liverpool and Broadgreen Clinical Research Facility, Liverpool University Hospitals NHS Foundation Trust, Cambridge University Hospitals NHS Foundation Trust, PHARMExcel, University Hospitals Birmingham NHS Foundation Trust, Guy’s and St Thomas’ NHS Foundation Trust, University College London, Portsmouth Hospitals University NHS Trust, NHS Greater Glasgow & Clyde, University of Cambridge, University College London Hospitals NHS Foundation Trust, University Hospitals Sussex NHS Foundation Trust, London Northwest University Healthcare NHS Trust, University Hospitals Dorset NHS Foundation Trust, Leeds Teaching Hospitals Trust and University of Leeds, Bangor University, University of Liverpool, University of Leicester, Bradford Institute for Health Research and Bradford Teaching Hospitals NHS Foundation Trust, Royal Devon and Exeter Hospital NHS Foundation Trust, Public Health Wales (Wrexham Maelor Hospital), MRC University of Glasgow Centre for Virus Research, UK Health Security Agency, University of Nottingham School of Medicine, UK.

The labels have been added to this press release as part of a project run by the Academy of Medical Sciences seeking to improve the communication of evidence. For more information, please see: if you have any questions or feedback, please contact The Lancet press office

[1] Quote direct from author and cannot be found in the text of the Article.
 [2] The MenACWY vaccine was used because it can cause similar minor side effects to ChAd and BNT, such as a sore arm, headache, and fever. Saline — which is often used as a control — does not cause any side effects.



Originally published at


Safety and immunogenicity of seven COVID-19 vaccines as a third dose (booster) following two doses of ChAdOx1 nCov-19 or BNT162b2 in the UK (COV-BOOST): a blinded, multicentre, randomised, controlled, phase 2 trial

Alasdair P S Munro, MRCPCH *, Leila Janani, PhD *, Victoria Cornelius, PhD *, Parvinder K Aley, PhD, Gavin Babbage, MPhil, Prof David Baxter, PhD, et al.

Open AccessPublished:December 02, 2021



Few data exist on the comparative safety and immunogenicity of different COVID-19 vaccines given as a third (booster) dose. To generate data to optimise selection of booster vaccines, we investigated the reactogenicity and immunogenicity of seven different COVID-19 vaccines as a third dose after two doses of ChAdOx1 nCov-19 (Oxford–AstraZeneca; hereafter referred to as ChAd) or BNT162b2 (Pfizer–BioNtech, hearafter referred to as BNT).


COV-BOOST is a multicentre, randomised, controlled, phase 2 trial of third dose booster vaccination against COVID-19. Participants were aged older than 30 years, and were at least 70 days post two doses of ChAd or at least 84 days post two doses of BNT primary COVID-19 immunisation course, with no history of laboratory-confirmed SARS-CoV-2 infection. 18 sites were split into three groups (A, B, and C). Within each site group (A, B, or C), participants were randomly assigned to an experimental vaccine or control. Group A received NVX-CoV2373 (Novavax; hereafter referred to as NVX), a half dose of NVX, ChAd, or quadrivalent meningococcal conjugate vaccine (MenACWY) control (1:1:1:1). Group B received BNT, VLA2001 (Valneva; hereafter referred to as VLA), a half dose of VLA, Ad26.COV2.S (Janssen; hereafter referred to as Ad26) or MenACWY (1:1:1:1:1). Group C received mRNA1273 (Moderna; hereafter referred to as m1273), CVnCov (CureVac; hereafter referred to as CVn), a half dose of BNT, or MenACWY (1:1:1:1). Participants and all investigatory staff were blinded to treatment allocation. Coprimary outcomes were safety and reactogenicity and immunogenicity of anti-spike IgG measured by ELISA. The primary analysis for immunogenicity was on a modified intention-to-treat basis; safety and reactogenicity were assessed in the intention-to-treat population. Secondary outcomes included assessment of viral neutralisation and cellular responses. This trial is registered with ISRCTN, number 73765130.


Between June 1 and June 30, 2021, 3498 people were screened. 2878 participants met eligibility criteria and received COVID-19 vaccine or control. The median ages of ChAd/ChAd-primed participants were 53 years (IQR 44–61) in the younger age group and 76 years (73–78) in the older age group. In the BNT/BNT-primed participants, the median ages were 51 years (41–59) in the younger age group and 78 years (75–82) in the older age group. In the ChAd/ChAD-primed group, 676 (46·7%) participants were female and 1380 (95·4%) were White, and in the BNT/BNT-primed group 770 (53·6%) participants were female and 1321 (91·9%) were White. Three vaccines showed overall increased reactogenicity: m1273 after ChAd/ChAd or BNT/BNT; and ChAd and Ad26 after BNT/BNT. For ChAd/ChAd-primed individuals, spike IgG geometric mean ratios (GMRs) between study vaccines and controls ranged from 1·8 (99% CI 1·5–2·3) in the half VLA group to 32·3 (24·8–42·0) in the m1273 group. GMRs for wild-type cellular responses compared with controls ranged from 1·1 (95% CI 0·7–1·6) for ChAd to 3·6 (2·4–5·5) for m1273. For BNT/BNT-primed individuals, spike IgG GMRs ranged from 1·3 (99% CI 1·0–1·5) in the half VLA group to 11·5 (9·4–14·1) in the m1273 group. GMRs for wild-type cellular responses compared with controls ranged from 1·0 (95% CI 0·7–1·6) for half VLA to 4·7 (3·1–7·1) for m1273. The results were similar between those aged 30–69 years and those aged 70 years and older. Fatigue and pain were the most common solicited local and systemic adverse events, experienced more in people aged 30–69 years than those aged 70 years or older. Serious adverse events were uncommon, similar in active vaccine and control groups. In total, there were 24 serious adverse events: five in the control group (two in control group A, three in control group B, and zero in control group C), two in Ad26, five in VLA, one in VLA-half, one in BNT, two in BNT-half, two in ChAd, one in CVn, two in NVX, two in NVX-half, and one in m1273.


All study vaccines boosted antibody and neutralising responses after ChAd/ChAd initial course and all except one after BNT/BNT, with no safety concerns. 

Substantial differences in humoral and cellular responses, and vaccine availability will influence policy choices for booster vaccination.


UK Vaccine Taskforce and National Institute for Health Research.

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