In a 23-page document (pdf) published to the agency’s website on Wednesday, FDA staff noted that the expected benefits of booster vaccinations depend entirely on the impact they have on “reducing disease relative to the primary series.”
The documents serve as a brief for the FDA’s Vaccines and Related Biological Products Advisory Committee, which is meeting on Sept. 17 to review Pfizer’s request to apply for a third “booster” dose of the vaccine in individuals aged 16 years and older.
“If the primary series of COMIRNATY [Pfizer Comirnaty vaccine] is still effective in preventing important COVID-19-related outcomes, then the benefit of booster vaccination is likely to be more limited than if effectiveness following the primary series has waned substantially,” FDA staff wrote.
“Factors supporting licensure of a booster dose should consider the effectiveness of primary vaccination with COMIRNATY over time and against circulating variants, the effectiveness (and its duration) of booster vaccination in preventing important COVID-19-related outcomes in individuals who have already received a primary vaccination series, the dynamics of the pandemic in the United States, and the risks of booster vaccination in the general population or in certain subpopulations,” staff continued.
The FDA staff declined to take a stance on whether or not to back booster shots, noting that regulators have not yet independently verified all the available data.
“There are many potentially relevant studies, but FDA has not independently reviewed or verified the underlying data or their conclusions,” staff wrote, noting that “some of these studies, including data from the vaccination program in Israel, will be summarized during the September 17, 2021 VRBPAC meeting.”
The staff noted that some observational studies have suggested a declining efficacy of the Pfizer vaccine over time against symptomatic infection or against the Delta variant, while others have not.
“However, overall, data indicate that currently US-licensed or authorized COVID-19 vaccines still afford protection against severe COVID-19 disease and death in the United States,” they wrote.
FDA staff also noted that while observational studies can enable understanding of real-world effectiveness, there are a “number of known and unknown biases that can affect their reliability” that can in turn make some studies more reliable than others.
Most importantly, data need to show that the benefit of a booster dose is weighed against any potential risk, and staff noted that it is currently not clear if those who receive a booster dose would have an increased risk of adverse reactions, such as myocarditis (inflammation of the heart muscle) or pericarditis (inflammation of the outer lining of the heart).
Since April 2021, increased cases of myocarditis and pericarditis have been reported in the United States following vaccination from Pfizer-BioNTech and Moderna vaccines, most notably among adolescents and young adults, according to the CDC.
“Furthermore, US-based studies of post-authorization effectiveness of BNT162b2 [the Pfizer-BioNTech vaccine] may most accurately represent vaccine effectiveness in the US population,” they added.
In a separate 53-page document (pdf) published on Wednesday, Pfizer stated that data from an ongoing, randomized, placebo-controlled study showed that a booster shot six months after the second dose of the vaccine showed it restored high levels of protection against COVID-19 outcomes by 95 percent.
President Joe Biden’s administration is pushing to begin distributing booster shots widely by Sept. 20 if both the FDA and Centers for Disease Control and Prevention agree.
However, a recent study from several top scientists at the World Health Organization and FDA found that the general population doesn’t need a booster dose.