Pfizer and BioNTech release on efficacy and safety up to six months after second vaccine dose, including data from South Africa

This article was published on
April 1, 2021

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Pfizer have published a press release stating that updated analysis of their Phase 3 study confirms high efficacy and no serious safety concerns up to 6 months following the second dose of their COVID-19 vaccine.

Pfizer have published a press release stating that updated analysis of their Phase 3 study confirms high efficacy and no serious safety concerns up to 6 months following the second dose of their COVID-19 vaccine.


What our experts say

Context and background


Media briefing

Media Release

Expert Comments: 

Prof Helen Fletcher

Is this robust data – is there enough data in the press release to be able to assess it?

This is great news.  From the numbers in this press release we can be confident that the efficacy of the Pfizer vaccine remains high up to 6 months following immunization in the US.  At 2-3 months of follow-up there were a total of 170 COVID-19 cases in the US trial with 8 in the vaccine group (Polack et al NEJM) and with up to 6 months of follow up there are 697 cases in the US trial with 50 in the vaccine group.

What does the data tell us about the effectiveness of this vaccine against the South African variant – is it possible to know?

The South African trial only started in October 2020 so the follow-up period is shorter and there are only 800 participants and 9 confirmed cases (all in the placebo group).  These findings should be interpreted with caution as the numbers are very small but there is reason for cautious optimism.

Any other comments about this data?

There was clearly a surge – at least a doubling – in the number of COVID-19 cases in the US trial in the second 3 months of follow-up when compared the first 3 months.  We need to see the data to confirm but it’s a stark reminder of the importance of getting COVID-19 vaccines developed, licensed and rolled out as fast as possible.

Penny Ward

Pfizer and BioNTech have presented updated information from the ongoing clinical efficacy study which continues to demonstrate a high level of protection against COVID 19 disease.  The group report data from 9 cases among 800 subjects in total recruited in South Africa.  All cases were in the placebo treated population and 6/9 of these were a result of infection with the B 1. 351 variant of concern.

Although it is encouraging that no clinically symptomatic cases of infection were detected in the (unreported) fraction of subjects vaccinated in South Africa, the information presented is insufficient to judge the level of clinical efficacy against this strain with any accuracy at this stage.

Similarly, without an understanding of the correlates of protection it is premature to compare potential for clinical effectiveness against illness caused by various viral strains based on in vitro comparisons.

In the meantime, it remains important to emphasize that the best vaccine is the one available immediately, all of which have been shown to prevent illness, hospitalisation and death resulting from infection by the strain(s) currently freely circulating in the UK.

Dr Peter English

Again, we only have a press release to go on.  I am sure we can believe what it says, but it would be preferable to see a peer-reviewed publication.

Based on the press release, however, this is excellent news.  The vaccine has been tested in a very large number of recipients: “44,000 participants 16 years of age and older, with more than 12,000 vaccinated participants having at least six months follow-up after their second dose”.  Apparently the data “[demonstrate] a favorable safety and tolerability profile”.  As ever, it would be nice to see more details, but this is reassuring.  The large number of participants means that the findings are likely to be very robust.

The vaccine was also effective: “…91.3% effective against COVID-19, measured seven days through up to six months after the second dose. The vaccine was 100% effective against severe disease as defined by the U.S. Centers for Disease Control and Prevention (CDC), and 95.3% effective against severe COVID-19 as defined by the U.S. Food and Drug Administration (FDA).”

With a prime-boost vaccine, you generally get a relatively slow rise in immunity following the initial, “priming” dose of vaccine.  After a second “booster” dose, however, you generally get a very rapid rise in antibody levels and efficacy, so “seven days after the second dose” is likely to be an appropriate time from which to have considered the efficacy of the dose to have kicked in.  It is possible that if they’d used 14 days from the second dose the efficacy would have been even greater.  Either way, these are very impressive data.

The 91.3% efficacy “against COVID-19” refers to efficacy against “confirmed symptomatic cases of COVID-19” – so it does not tell us about efficacy against the asymptomatic cases that we now know are responsible for a lot of the transmission.  This is not a criticism of the study: phase III studies do not generally report on this.  But it will be important to know how effective the vaccine is at preventing infection and transmission, if we are to be able to achieve herd immunity through vaccination.

The study pools data from various different study cohorts, including one in South Africa, where the B.1.351 lineage is prevalent.  There have been concerns that vaccines may not be as effective against this variant, so it is reassuring that in the 800 participants enrolled there, all nine cases of COVID-19 were in the placebo group, and none in the vaccinated group.  Nine is not a huge number, so we cannot be sure of the precise efficacy against this variant (no vaccine is 100% effective); but it is extremely reassuring.

Prof Peter Openshaw

This new press release shows limited data with respect to the ability of the Pfizer vaccine to protect against severe disease, even that caused by the South African variant virus B.1.351.  There is clearly much more data to come from this study and what has been announced is just focussed on the severe disease only.  The information they provide is in line with the known effects of highly immunogenic vaccines – i.e. those that induce a very high level of antibody – that these vaccines protect against severe disease even if the B.1.351 variant is circulating.

The number of cases is small (800 participants were enrolled; nine cases of COVID-19 were observed, all in the placebo group) so the confidence interval is wide.  It might be 100% effective, but as more data accumulates it is possible that cases will appear that show a lower protection rate.  It does not tell us if the vaccine also prevents virus circulation in the community but it seems likely that it will reduce less severe types of infection as well.  When the full data is released, this will be a key factor to consider.


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