More People Opting for Boosters Amid Scientific Uncertainty

Lindsay Kalter

August 05, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Infectious disease and pharmacy specialist Jason Gallagher received his one-and-done Johnson & Johnson COVID-19 shot in November. But as he prepared to travel to the high-transmission state of Florida in June, he managed to also get a booster shot of the Pfizer vaccine to help stave off the highly infectious Delta variant.

"It isn't that I thought the J&J shot wouldn't work. There just wasn't much data on its effectiveness against the Delta variant," says Gallagher, PharmD, a professor at Temple University in Philadelphia. "I was traveling — to Florida, no less — and wanted to be extra cautious."

Gallagher is one of a growing number of people choosing to mix COVID-19 vaccines to boost their previous doses, as researchers still work to determine if — and when — boosters may be necessary.

Though scientists are getting closer to an answer, the consensus remains the same: There just isn't enough information yet.

study from Pfizer and BioNTech researchers reports a slow decline in efficacy in the first 6 months after the second dose of the two-shot vaccine. The vaccine starts out with an efficacy rate of about 96% against symptomatic disease for the first 2 months — a number that falls about 6% every 2 months after that. The efficacy is about 84% after 4 to 6 months.

The new data comes on the heels of a study from Israel's Health Ministry showing waning vaccine effectiveness as the Delta variant spreads, which has prompted the country's health officials to consider giving booster shots to elderly residents. The study shows the two-shot Pfizer vaccine is only 39% effective in preventing an infection — half as effective it was shown to be 2 weeks ago, and far less than the 95% when it was released in 2020.

But some experts have questioned the accuracy of these results and note that the vaccine is still highly effective in preventing severe illness.

"There's just no way of rushing the data. We will have that information at some point," he says. "There's no use in taking away resources from people who aren't vaccinated yet."

But some are not convinced. As many, like Gallagher, call for another dose, the city and county of San Francisco announced that anyone who received the Johnson & Johnson one-dose vaccine may get a booster shot of either the Pfizer or Moderna vaccine.

Many scientists do believe that boosters can help some people. Kuritzkes notes those who are immunocompromised, and mounted a weak initial immune response to the vaccine, are likely to benefit from a booster. The Advisory Committee on Immunization Practices (ACIP) came to the same conclusion in July, but the group does not have regulatory authority and can only make recommendations.

Otto Yang, MD, a professor of medicine and associate chief of infectious diseases at UCLA Health, says the data so far is "pointing very strongly" in favor of boosters — particularly given what scientists know about other types of coronaviruses. But, he says, it depends on how you define "protection."

For example, there is protection from symptomatic infection, and then there is protection from serious illness and death. Generally, antibodies are responsible for the first, and T cells are responsible for the latter. Because T cells tend to last longer than antibodies, Yang says it is likely that the vaccine will protect against serious illness longer than symptomatic infection.

"It's really hard to say when people will need them. I'm guessing somewhere in the range of 6 months to a year of when they get their shot," he says. But, he says, though there is no reason to think a booster would be unsafe, there is still no data on that. "Right now, that's venturing into the unknown."

Gallagher based his decision in part on data from the AstraZeneca shot — which uses a similar vaccine platform as J&J — showing it was safe and effective in combination with a Pfizer booster.

While Gallagher declined to say how he was able to get a booster, he says he's glad he did.

 "I wasn't willing to sit around and wait," he says.


MedRxiv: "Six Month Safety and Efficacy of the BNT162b2 mRNA COVID-19 Vaccine."

Government of Israel.

Jason Gallagher, PharmD, professor, Temple University, Philadelphia.

Daniel Kuritzkes, MD, chief, Division of Infectious Diseases, Brigham and Women's Hospital, Boston.

Otto Yang, MD, professor of medicine, associate chief of infectious diseases, UCLA Health.