Claims that Pfizer vaccine wasn’t tested on preventing transmission need context

24 October 2022
What was claimed

A Pfizer executive has admitted the company did not test whether its Covid-19 vaccine stopped transmission of the virus before its launch.

Our verdict

Pfizer was always clear it did not test whether the vaccines reduced the risk of transmission among already-infected individuals. But the trial did show the vaccines reduced infection risk in the first place, so reduced the risk of onward infection.

“In COVID hearing, #Pfizer director admits: #vaccine was never tested on preventing transmission.”

A claim by a member of the European Parliament (MEP) that an executive at Pfizer has revealed that Covid-19 vaccines weren’t tested for their impact on transmission is missing important context.

On 11 October, Dutch MEP Rob Roos tweeted a video clip from part of a Covid hearing at the European Parliament the previous day where he asked Janine Small, president of international markets at Pfizer, whether its Covid-19 vaccine was tested for its impact on transmission prior to its release. 

Ms Small confirmed it was not tested in that way before release.

The video continues with Mr Roos addressing the camera to say: “This is scandalous. Millions of people worldwide felt forced to get vaccinated because of the myth that you do it for others. Now this turned out to be a cheap lie. This should be exposed."

At the time of writing, the post has been viewed more than 13 million times and retweeted more than 178,000 times. His comments have also attracted support from several prominent figures .

In an email to Full Fact, Mr Roos added: “Governments worldwide have introduced Covid mandates and passports that had an enormous impact on millions of people. They did so by explicitly arguing that vaccinated people cause less transmission of the virus. Ms. Small's response to my question proves this was an assumption by governments for which no evidence had been provided.”

However, Pfizer was not required to test the impact of its vaccine on transmission prior to its release. 

A spokesperson from Pfizer told Full Fact by email: “The primary endpoints of our Covid-19 vaccine study were to evaluate the efficacy of the vaccine candidate in preventing Covid-19 disease in participants who had not been infected with the SARS-CoV-2 virus before they received the vaccine […]The trials were not designed to evaluate the vaccine’s effectiveness against transmission of SARS-CoV-2.”

The fact that the Covid-19 vaccines did have an impact on transmission, particularly against the earliest variants, was verified with real world data after the vaccines were rolled out. Although the impact of vaccines on transmission has lessened against the Omicron variant, this does not mean that the vaccines impacting transmission was just a “myth” or “assumption”.

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Transmission was not tested for

Despite Mr Roos presenting this as “breaking news”, Pfizer did not claim to have studied the issue before the vaccine’s market release.

Furthermore, the fact that the vaccine’s impact on transmission had not been assessed was already in the public domain.  

On 3 December 2020, the day after the UK became the first country in the world to approve Pfizer's vaccine, CEO Albert Bourla told NBC News that the company had not investigated whether vaccinated people who became infected with the virus could pass it on to others. 

He said: “I think this is something that needs to be examined. We are not certain about that right now.”

Similarly, when the US Food and Drug Administration (FDA) issued emergency approval for the Pfizer Covid vaccine on 11 December 2020 it noted there was no “evidence that the vaccine prevents transmission of SARS-CoV-2 from person to person.”

During a virtual meeting hosted by the European Medicines Agency on that same day, to discuss the development of all Covid vaccines, one presentation said the question of transmission remained “uncertain at approval" and would require "specific studies post approval".

In January 2021, the World Health Organisation (WHO) issued guidelines on the use of the Pfizer vaccine and noted there was “not yet any evidence of the effect of the vaccine on virus transmission”, recommending that other measures such as mask wearing and hand washing be continued to slow the spread of Covid. 

The same WHO document also lists areas of research it recommends be pursued after the vaccine has been authorised. They include “studies to demonstrate whether this vaccine reduces SARS-CoV-2 transmission and viral shedding”.

Such reports make it clear Pfizer had not carried out tests to determine whether the vaccine stopped transmission prior to it being authorised, nor claimed to. 

Pfizer’s clinical trial results show the main measure analysed to determine whether the vaccine worked was symptomatic cases of lab-confirmed Covid-19. In the trial there were eight cases among vaccinated trialists and 162 among those given the placebo, indicating the vaccine was 95% effective. 

Impact on transmission

It’s important to note the difference between the technical definition of transmission used by Pfizer, and a more colloquial understanding of infection.

Pfizer’s trial did not look at whether or not infected people were more or less likely to pass the virus on to someone else, based on whether they were vaccinated or not. 

But Pfizer’s trial did show that vaccination would lower your risk of being infected in the first place, and so would reduce the level of transmission. As the UK Health Security Agency said in its January 2022 vaccine surveillance report: “Uninfected individuals cannot transmit; therefore, the vaccines are also effective at preventing transmission.”

What we know now about transmission

Following the vaccine roll-out, some early real-world studies in the UK and Israel did seem to indicate that the vaccines reduced levels of transmission from those infected with the Alpha variant. 

Later studies found the vaccines were less effective at preventing the transmission of subsequent variants of Covid-19 such as Delta and Omicron

A study by the UK Health Security Agency found that two doses of the Pfizer vaccine initially reduced the transmission of the Delta variant by 31%. 

A subsequent study of transmission of the Omicron variant in Denmark, reported in Nature,  found that, due to its ability to evade prior immunity, vaccines were “less likely to be sufficient to curb transmission with Omicron compared to previous variants”.

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