Researchers Find ‘Healthy Vaccinee Bias’ May Have Led to Inflated Efficacy Claims of Pfizer’s COVID-19 Booster

In a Letter to the Editor published in the New England Journal of Medicine (NEJM) on July 20, epidemiologist Dr. Tracy Høeg, physician-scientist Dr. Vinay Prasad, and Dr. Ram Duriseti from Stanford School of Medicine stated that a 2021 Israeli study on Pfizer’s COVID-19 booster may have overestimated the vaccine’s effectiveness by concluding those who received a booster had 90 percent lower mortality due to COVID-19 than non-boosted participants.

The group analyzed deaths attributed to COVID-19 in both the 2021 study and in a later statement by the same study authors to estimate mortality not associated with the virus based on a person’s vaccination status. The mortality unrelated to COVID-19 in the boosted group was compared to mortality unrelated to COVID-19 in the non-boosted group.

The authors found a 94.8 percent lower mortality unrelated to COVID-19 among participants in the boosted group and a “markedly lower incidence of adverse health outcomes” in the same group. Additionally, COVID-19 mortality, whether boosted or not, was essentially the same in the 2021 study, generating a “strong concern” vaccine efficacy was overestimated.

In other words, an individual’s underlying health plays a significant role in COVID-19-related mortality. If the boosted group had fewer health conditions that weren’t accounted for in the data, it wouldn’t be accurate to attribute lower mortality to a third dose of Pfizer’s vaccine.

In a July 20 statement on Twitter, Dr. Høeg said she and her co-authors showed strong evidence of “healthy vaccinee bias” in the Israeli study on boosters that wasn’t disclosed, which could explain “all reported benefits of the booster.”

“No study is perfect, but this one should be retracted,” Florida Surgeon General Dr. Joseph A. Ladapo said in a statement on Twitter. Dr. Ladapo congratulated the letter’s authors for showing that the 90 percent benefit attributed to boosters in the NEJM’s study was “entirely due to selection bias” and demanded randomized controlled trials.

Although the authors said Israel has not been transparent with the data used in their studies, they believe “healthy vaccinee bias” may also have led to overestimates of vaccine effectiveness in other studies.

Israeli Study Background

The Letter to the Editor was written in response to the study published on Dec. 23, 2021, in The NEJM, initially looking at whether a third dose of Pfizer’s vaccine effectively reduced COVID-19 mortality. The Israeli Ministry of Health had already approved a booster dose on July 30, 2021, but evidence that an additional dose lowered mortality was lacking.

Researchers obtained data from Clalit Health Services of 843,208 individuals over 50 years of age who received two Pfizer vaccine doses at least five months before the start of the study. They then compared mortality due to COVID-19 among participants who received a booster during the 54-day study period to non-boosted participants.

According to the study, 65 participants in the boosted group died due to COVID-19 compared to 137 participants in the non-boosted group. However, in response to another Letter to the Editor published in February 2022, the study’s authors provided additional study details.

The authors stated 506 deaths occurred in the boosted group during the study period—441 unrelated to COVID-19 and 65 deaths related to COVID-19—compared to 1,100 deaths in the non-boosted group, 963 of which were related to COVID-19 and 137 that were unrelated. They also stated deaths unrelated to COVID-19 should be “interpreted with caution” because they were not adjusted for the numerous factors that could affect all-cause mortality.

In a July 20 tweet, Dr. Prasad said he and his co-authors proved the results of the Israeli studies were distorted and boosters could not have reduced non-COVID-19 deaths.

“Israeli authors concede this in reply,” Dr. Parad said. “Wow! Millions got unproven boosters. FDA failed.”

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