Post claiming the Pfizer vaccine has only a 12% efficacy rate is inaccurate

A Facebook post has claimed that new documents released by Pfizer show that its Covid-19 vaccine has only a 12% efficacy rate, caused an “alarming amount of still births and deaths in pregnant women”, and is not recommended by the manufacturer for pregnant or breastfeeding women.

These claims are false and are based on misinterpretations of documents that were released over a year ago, not as part of a recently released collection of documents about Pfizer’s clinical trials. 

Claims that Pfizer’s vaccine is only 12% effective are inaccurate

This Facebook post is one of many similar posts that have been circulating on social media in recent days stating that the efficacy rate of Pfizer’s Covid-19 vaccine is far lower than advertised.

The claim does not appear to be based on anything contained in the batch of documents recently made public by the US Food and Drug Administration (FDA), which some campaigners against the vaccine are referring to as the “Pfizer data dump,” or “Pfizer docs.”

While we can’t say for certain from where this particular Facebook post sourced its claim, similar references to the 12% efficacy figure seem to have been based on a briefing document about a large-scale clinical trial of Pfizer’s Covid-19 vaccine published by the FDA and dated 10 December 2020.

Epidemiologist Gideon Meyerowitz-Katz and biomedical data scientist Jeffrey Morris have both linked the spread of this inaccurate figure to a Substack post which quotes the FDA document, and compared the number of “suspected but unconfirmed” Covid-19 cases among vaccinated and unvaccinated trial participants.

The FDA document in question reads: “Among 3410 total cases of suspected but unconfirmed COVID-19 in the overall study population, 1594 occurred in the vaccine group vs. 1816 in the placebo group”.

If these cases were in fact all the genuine Covid-19 cases identified in the trial, this would indeed have demonstrated a 12% efficacy rate. However, the figures include some that were likely not positive and exclude some genuine cases.

The 12% claim relies on an incorrect interpretation of the meaning of “suspected but unconfirmed” Covid-19 cases, which the FDA briefing document also refers to as “suspected cases of symptomatic COVID-19 that were not PCR confirmed.”

It appears that Pfizer’s definition of a “suspected but unconfirmed” Covid-19 case was someone who experienced a Covid-19 symptom, but subsequently tested negative through a PCR test. 

Therefore this data would include many cases that were not actually Covid-19 and exclude cases that were Covid-19 and confirmed by a PCR test. 

It therefore can’t be used to calculate the vaccine’s efficacy against symptomatic Covid-19, which the trial was designed to measure.

What is the true efficacy rate?

Initial large scale clinical trials found that the efficacy of the Pfizer vaccine was 95% among trial participants who had not previously tested positive for Covid-19, and 94.6% for both those who had and who had not tested positive prior to 7 days after receiving their second dose.

The trials were done prior to the emergence of variants of concern, some of which evade the protection from vaccines more effectively, and prior to the widespread roll-out of booster vaccinations. This means the current effectiveness of the Pfizer vaccine may not match the efficacy observed during the trials. 

The UK Health Security Agency (UKHSA) indicates that the Pfizer vaccine is less effective at preventing symptomatic illness with the Omicron variant than it was with the original SARS-CoV-2 virus, with protection waning over time.

A report published by the UKHSA on 12 May 2022 concerning vaccine effectiveness against symptomatic illness states: “After 2 doses of the AstraZeneca vaccine, vaccine effectiveness against the Omicron variant starts at 45 to 50% then drops to almost no effect from 25 weeks after the second dose. 

“With 2 doses of Pfizer or Moderna effectiveness dropped from around 65 to 70% down to around 15% by 25 weeks after the second dose. 

“Two to 4 weeks after a booster dose of either the Pfizer or Moderna vaccine following an AstraZeneca or Pfizer primary course, effectiveness ranges from around 60 to 75%, dropping to almost no effect from 20+ weeks after the booster. Vaccine effectiveness estimates for the booster dose are very similar, irrespective of the primary course received.”  

The vaccine’s effectiveness against preventing hospitalisation or death from the Omicron variant is estimated to be significantly higher than its effectiveness against preventing symptomatic illness. 

‘No safety concerns’ during pregnancy and breastfeeding

The Facebook post also claims that the documents reveal “an alarming amount of still births and deaths in pregnant women”.

As we have written before, pregnant women were excluded from initial large scale Covid-19 vaccine trials (though some became pregnant over the course of the trials). This means the FDA document about the trial, from which the 12% claim originates, contains little information about the vaccine’s effect on pregnant women.  

It notes that two adverse events related to pregnancy, spontaneous abortion and retained products of conception, occured in the placebo group, so those women had not received the Pfizer vaccine.

If the post’s author was referring to the ‘Pfizer data dump’, this does contain details on a number of adverse events reported by pregnant women after vaccination. But as Reuters reported it is difficult to assess whether the vaccine was responsible for any of these or whether the rate of, say, still births among vaccinated women was different from the rate among unvaccinated women. 

There have however since been a number of studies aimed at identifying any problems related to the Covid-19 vaccine during pregnancy, from which no significant safety concerns have arisen. 

The post also claims that Pfizer did not recommend the vaccine during pregnancy or breastfeeding. This may refer to misinformation we recently wrote about regarding a document published in December 2020 by the Medicines and Healthcare Products Regulatory Agency (MHRA).

The document states that “COVID-19 mRNA Vaccine BNT162b2 [the Pfizer Covid-19 vaccine] is not recommended during pregnancy” and “it is unknown whether COVID-19 mRNA Vaccine BNT162b2 is excreted in human milk” therefore “a risk to the newborns/infants cannot be excluded.”

A spokesperson for the MHRA confirmed to Reuters that the document reflected the organisation’s “assessment at the time.”

However, they added: “Since then new data which has come to light (both non-clinical and post-authorisation ‘real world’ data) supports the updated advice on vaccinating those who are pregnant and breastfeeding.”

“Over 104,000 pregnant people have received at least one dose of COVID-19 vaccine in England and Scotland and no concerns of the safety of the vaccines have been raised.”

The MHRA’s current guidance recommends that the vaccine should only be considered if the potential benefits outweigh the risks to the mother and foetus.

Both the NHS and the Royal College of Obstetricians and Gynaecologists (RCOG) say that the vaccine is “strongly recommended” for people who are pregnant or breastfeeding. The RCOG has published evidence that suggests pregnancy increases the risk of becoming seriously ill if you catch Covid-19, with the NHS saying Covid-19 is also linked to increased risk of complications during pregnancy.

Image courtesy of the CDC.