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In a viewpoint published in the Lancet, leading health officials from around the world say booster doses of the COVID-19 vaccine aren’t necessary yet.

“Current evidence does not … appear to show a need for boosting in the general population, in which [vaccine] efficacy against severe disease remains high,” the authors write.

The health authorities say some people, including those with weakened immune systems, may need booster doses to heighten their protection against COVID-19, but that data supporting the need to give the general population additional doses aren’t convincing at this point.

The commentary included experts from the U.S. Food and Drug Administration (FDA) and the World Health Organization (WHO), as well as major academic institutions around the world. It heightens the ongoing tension among public health officials concerning if, and when, booster doses should be given. One issue surrounds the science—public health authorities are still interpreting data on infections and disease among vaccinated people to understand what those mean for immunity. Another issue is the limited supply of vaccines for most of the world. Earlier this summer, the WHO asked for a moratorium on providing boosters, at least until the end of the year, until more people, especially in lower resource countries, can get vaccinated. The U.S.’s public health leaders, however, led by the White House, decided to roll out boosters beginning Sept. 20, despite the fact that such additional doses have not been deemed safe or effective yet by the FDA.

Studies show that protection against COVID-19, measured by the level of antibodies generated by people who are vaccinated, wanes after about six months. But that doesn’t mean those people are dramatically more vulnerable to disease, the authors say. “Reductions in neutralizing antibody titer do not necessarily predict reductions in vaccine efficacy over time, and reductions in vaccine efficacy against mild disease do not necessarily predict reductions in the (typically higher) efficacy against severe disease.” Even against the latest, more transmissible variants of SARS-CoV-2, including Delta, current vaccines continue to protect people from getting severe COVID-19, these health officials say. While rare, studies have reported breakthrough infections in which vaccinated people have tested positive for the virus. In the U.S., Centers for Disease Control (CDC) data show that between January and April 2020, around 10,000 breakthrough infections were voluntarily reported by states among 101 million vaccinated people (a rate of 0.01%). About a quarter of those people experienced no symptoms and the majority of those that did were mostly mild and did not require hospitalization or critical care.

So why have some health officials so strongly advocated booster shots, including the top public health leaders in the U.S.? In August, President Biden’s health team, which includes chief medical advisor Dr. Anthony Fauci, the heads of the National Institutes of Health, the CDC and the FDA, announced that the U.S. plans to start distributing booster doses on Sept. 20 for anyone eight months out from their last vaccine dose. The announcement surprised many in the medical community, since any additional dose would have to be authorized or approved by the FDA first, and then recommended by the CDC. While both Pfizer-BioNTech and Moderna have submitted requests to the FDA to greenlight a booster, the agency has not made a decision yet. The agency did, in August, authorize booster shots for a limited population of people with compromised immune systems. The FDA Vaccines and Related Biological Products Advisory Committee meets on Sept. 17 to discuss the data submitted by Pfizer-BioNTech (a similar meeting for Moderna, if it will occur, has not been scheduled yet).

The White House said it made its decision to roll out booster shots in order to give states and other local health departments time to prepare for another massive vaccination campaign—“to stay ahead of this virus by being prepared,” Surgeon General Dr. Vivek Murthy said during a briefing on Aug. 18. The team also said data concerning breakthrough infections among vaccinated people were concerning, and pointed in the direction of waning immunity.

But the Lancet authors note that interpreting those data can be tricky. For example, much-cited research from Israel shows dropping levels of immunity and higher rates of breakthrough infections in recent months. However, that research shows breakthrough infections were higher among people vaccinated in January and April than those vaccinated in February and March, which doesn’t seem to follow any obvious scientific rationale.

Even if a booster does is needed, the authors say that a more targeted vaccine against specific variants might be more effective than adding an additional dose of the existing vaccine.

The authors also argue that using limited supplies of existing vaccines around the world to first vaccinate the unvaccinated would do more to reduce reduce the emergence of new variants and raise overall immunity rates than would boosting the already vaccinated. Although existing vaccines may not help people avoid getting infected to begin with, they continue to provide adequate protection against severe disease. Before boosters are rolled out, they say, more data are needed to justify which types of doses are most effective, when they might be needed, and for whom.

“Any decisions about the need for boosting or timing of boosting should be based on careful analyses of adequately controlled clinical or epidemiological data, or both, indicating a persistent and meaningful reduction in severe disease,” write the authors, “along with evidence about whether a specific boosting regimen is likely to be safe and effective against currently circulating variants.”

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