Monday, August 2, 2021

COVID vaccine boosters: now, later, or never?

 Jennifer Middleton, MD, MPH

As COVID-19 infection numbers continue to rise, and with continued breakthrough cases in persons who have been fully vaccinated, questions about booster shots are circulating. The delta variant is wreaking havoc in many places across the world, especially in pockets of the US with low vaccination rates including Missouri, Arkansas, Louisiana, and Florida. Reports of breakthrough infections in persons fully vaccinated have contributed to many alarming headlines, though public health experts have been quick to emphasize that the vast majority of severe infections are occurring in unvaccinated persons.

Data from Israel, which has vaccinated 60% of its population against COVID-19, is fueling fears as well. Estimates that the vaccine was just 39% effective at preventing infection in the last couple of months there have been heavily publicized, though Israeli researchers have been quick to point out that the vaccine has consistently remained highly effective (>90%) at preventing severe COVID-19 infection:

Experts suggested several possible explanations for the seeming decline, including the possible waning of immune protection in people vaccinated early in the year....[b]ut it's also possible that the apparent decline is a mathematical fluke. Case numbers are much lower in Israel now that they were earlier in the year..."I think that data should be taken very cautiously because of small numbers," [according to] Eran Segal, a biologist at the Weizmann Institute of Science.

With these concerns swirling, Pfizer announced preliminary results regarding its investigation into the efficacy of a 3rd COVID-19 vaccine dose, finding "[p]ost dose 3 [antibody] titers vs. the Delta variant are >5-fold post dose 2 titers in 18-55 [year-olds] & >11-fold post dose 2 titers in 65-85 [year-olds]." These numbers sound impressive, but this antibody titer study was only conducted in 23 individuals, and it's unclear precisely how antibody titers correlate with more important patient-oriented outcomes such as severe illness and death. In a joint statement issued earlier this month, the FDA and the CDC asserted that "Americans who have been fully vaccinated do not need a booster shot at this time....We are prepared for booster doses if and when the science demonstrates that they are needed." The stark inequity of worldwide vaccine distribution also raises ethical concerns regarding devoting more vaccine product to the US when most of the world's population still does not have access to even a first COVID-19 vaccine dose.

The Advisory Committee of Immunization Practices (ACIP) has been discussing the possible benefit of a booster dose for immunocompromised persons, though ACIP declined to make any recommendations at its meeting last week. While we wait for more data and possible recommendations on boosters, the CDC is now advising all Americans, even those who have been fully vaccinated, to mask indoors if they are in areas of high COVID-19 transmission (which is most of the US as of this writing), especially if they live with persons unable to be vaccinated and/or at higher risk of complications from COVID-19 disease. We should still continue to advise our patients, even our vaccinated ones, to exercise caution and thoughtfulness as the delta variant continues to spread.