Israel, which got an early start on vaccinating its population, began offering third doses of the Pfizer–BioNTech vaccine in July, to people aged 60 and over. The latest analysis links the third jab not only with a significant reduction in severe COVID-19, but also with an 11.3-fold reduction in SARS-CoV-2 infections.
But Ellie Murray, an epidemiologist at Boston University in Massachusetts, cautions that observational studies such as this analysis can contain biases that are difficult to identify and account for. For example, people who sign up to get a booster might have a different risk of COVID-19, or behave differently, from people who do not get a third jab.
Ellenberg says that the authors try to address some of these potential biases. Even if not all biases have been eliminated, she says, the magnitude of the effect suggests that the booster offers some protection, at least in the short term. The authors of the study could not be reached before publication.
The findings come as a slew of wealthier nations consider offering booster shots. An advisory committee of the US Food and Drug Administration (FDA) will discuss Pfizer’s application to supply boosters in the United States on 17 September. One of the authors of the Israeli study is slated to present data to the committee.
Murray argues that the potential biases in the data, and insufficient evidence for waning immunity after vaccination, mean that the latest findings don’t indicate a “strong need” for boosters. “From a public-health perspective, it’s way, way more impactful to get more people vaccinated than it is to boost the vaccine effectiveness by a few percentage points in those who have already gotten the vaccine,” she says.
Murray is not alone in finding the Israeli results insufficient to justify boosters. A review2 published on 13 September by a team that includes two high-ranking FDA scientists cites a preprint3 of the study and notes that the short-term protective effect documented in Israel “would not necessarily imply worthwhile long-term benefit”.
Dvir Aran, a biomedical data scientist at Technion — Israel Institute of Technology in Haifa, says that Israel has deployed boosters to stop transmission in younger people and to prevent severe disease and deaths in older people.
“Is it the best way? Whether a two-week lockdown would have given a similar result, I can’t answer that question,” he says. “But it’s an interesting approach, trying to stop an outbreak like this with vaccinations.”