There was no discussion Wednesday of any potential side effects of a third dose.
So what do we know about whether healthy, fully vaccinated people should get a booster? Here are answers to seven key questions.
1. What evidence are vaccine makers giving federal regulators to support the idea that an additional shot is needed?
It’s unclear how the booster may be authorized by regulators. On Tuesday, FDA spokesperson Abby Capobianco said federal agencies are reviewing laboratory and clinical trial data as well as data from the real world. Some data will come from specific pharmaceutical companies, but the agency’s analysis “does not rely on those data exclusively,” she said.
The companies, for their part, are racing to produce data. On Monday, Pfizer and BioNTech submitted initial but promising results from a phase 1 study of the safety and immune response from a booster dose given at least six months after the second dose. Late-stage trial results that evaluate the effectiveness of a third dose are “expected shortly,” Pfizer spokesperson Jerica Pitts confirmed this week.
Moderna President Stephen Hoge said during his company’s earnings call this month that a third dose is “likely to be necessary” this fall because of the highly contagious delta variant. Moderna spokesperson Ray Jordan said Tuesday the company is in talks with regulators but hasn’t provided an estimated timeline.
Johnson & Johnson, whose vaccine is administered in a single shot, hopes to share results soon from a late-stage clinical trial studying the safety and efficacy of a two-dose regimen in 30,000 adults. The study is looking at “potential incremental benefits” with a second dose, company spokesperson Richard Ferreira wrote in a Tuesday email.
2. Why might healthy people not need a booster yet?
Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the National Institutes of Health and FDA, said current federal guidance does not recommend a booster and there’s no “science-based” reason to get an additional shot at this time — even after receiving the J&J vaccine.
The current mRNA vaccines work by inducing a certain level of neutralizing, virus-specific antibodies with the first dose. Then the second dose brings on an exponential increase in the measurable level of specific neutralizing antibodies — and, more important, there’s evidence that the second dose of mRNA vaccine also gives cellular immunity, Offit said.
“That predicts relatively longer-term protection against severe critical disease,” he said. A single dose of the J&J vaccine — which uses a different technology, called an adenovirus vector — has been shown to provide the equivalent response to the second dose of an mRNA vaccine, he said.
3. How do the three vaccines authorized in the U.S. compare?
A recent preprint — a paper that has not been peer-reviewed — from the Mayo Clinic suggests that the Moderna vaccine may be more protective against the delta variant than the Pfizer-BioNTech vaccine. However, that research is based on examining the vaccination history of thousands of people who got covid, rather than a direct comparison of the vaccines, said Dr. Catherine Blish, a specialist in infectious diseases at Stanford Medicine.
“I would be hesitant to alter any practices or change behavior in any way based on that data,” she said.
The Moderna and Pfizer-BioNTech vaccines are administered differently, which could factor into how much mRNA the body receives to code into protein, said Dr. Monica Gandhi, a specialist in infectious diseases at the University of California-San Francisco. Moderna’s dosing is two shots of 100 micrograms delivered four weeks apart, while the Pfizer-BioNTech vaccine’s two 30-microgram doses are delivered three weeks apart.
At the end of July, Pfizer and BioNTech announced findings that four to six months after a second dose their vaccine’s efficacy dropped from a peak of 96% to about 84%. With its own data of fading efficacy, the Israeli government launched a vaccination campaign this month encouraging more than 1 million residents over age 50 to get a third shot.
As for J&J’s one-shot vaccine, there’s no evidence that recipients are being hospitalized with breakthrough infections at a higher rate than if they had received other vaccines, said Dr. Amesh Adalja, a specialist in infectious diseases at Johns Hopkins Center for Health Security.
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