Amid a nationwide campaign to promote COVID-19 booster shots, vaccine scientists and public health experts say vaccines are still holding up remarkably well for most people -- depending on how effectiveness is measured.
In fact, many scientists now worry that the recent booster shot authorization could give the false impression that existing vaccines are no longer offering protection.
"They all work well," said Dr. Paul Goepfert, an infectious disease physician and director of the Alabama Vaccine Research Clinic. "They aren't perfect by any means. But if your bar is prevention of hospitalizations in the United States, they still work incredibly well."
A vaccine's effectiveness can be measured in several different ways. One is their ability to protect people from mild infections. When first authorized, Pfizer and Moderna's vaccines proved 95% and 94% effective using this threshold, and Johnson & Johnson's single-shot vaccine proved 75% effective.
"No vaccine entirely prevents disease," said Dr. Anna Durbin, an associate professor at the Johns Hopkins University School of Medicine.
Even protecting from mild infections is "a high bar for a vaccine," said Dr. Paul Offit, an FDA advisory panel member and director of the Vaccine Education Center at Children's Hospital of Philadelphia.
And even if a vaccine achieves that high bar, it quickly starts waning. Antibodies, which protect against infection, surge after vaccination -- but then rapidly fade. Other parts of the immune system, like T cells and B cells, remain more stable over time, protecting against severe disease and death.
Scientists and public health experts say what really matters is a vaccine's ability to prevent severe illness and hospitalization. And on that metric, all three vaccines performed well from the start -- each more than 90% effective -- and have remained relatively stable, even through the emergence of a new delta variant
"It's the unusual vaccine that protects you against mild illness," Offit said. "It's OK to get infected. It really is. You just don't want to get seriously infected."
A comprehensive study from New York state offers a glimpse of this phenomenon, finding that all three vaccines remained roughly 86% effective when it came to reducing the risk of being hospitalized with COVID-19 from May to August.
But over the same time frame, all three lost some ability to protect against breakthrough infection. Though vaccine efficacy started from a high point, from May to August, efficacy of the Pfizer vaccine declined by 25% to 14% depending on age, the Moderna vaccine declined 18% to 9% and the Johnson & Johnson vaccine declined 19% to 11%.
When it comes to booster shots, experts agree there are some people who clearly need them -- chiefly, people with weakened immune systems and the elderly, who also mount a less robust immune response.
Today, more than 13 million people in the United States have already received a booster shot. Moderna and Pfizer vaccines are authorized six months after the first shot for those 65 and older, and those at high risk of developing COVID-19. Johnson & Johnson boosters, meanwhile, are authorized for anyone at least two months after the first shot. After the Pfizer booster shots were authorized, CDC director Rochelle Walensky said it was a "walk, don't run" situation, during a conversation with The Atlantic.
"There is no doubt that if you were to get a booster every two months or so, you may prevent all symptoms," Durbin said. "But there's a cost to that."
It's expensive, for one. And there are concerns that frequent boosting could dull the immune system's ability to fight future variants, because the boosters could focus the immune response on the COVID-19 strain used to make the current vaccines.
Widely publicized concerns about breakthrough infections may have "focused the conversation prematurely on the need for boosters," said Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and an ABC News contributor.
"Concerns about waning immunity and breakthrough cases have likely been overblown," Brownstein said.
And boosters for the vaccinated -- while offering a temporary shield against mild infection -- are unlikely to dramatically turn the tide of the pandemic.
"Boosting is not going to be what's going to be the issue for us as a country," said Goepfert. "It's finding the people who are still unvaccinated."