But David Dowdy, an infectious disease epidemiologist at Johns Hopkins University, notes that the apparent decline in protection could have other causes, including changes in individual behavior and the rate of transmission in the community. Dowdy notes that in the New York study, the efficacy of COVID-19 vaccines dropped most in the 18- to 49-year-olds and least in those older than 65. That suggests an increase in risky behavior among younger people—such as visits to restaurants, bars, and concerts—may also account for the trend. “People’s behavior has changed substantially” since the last wave, he says, with fewer masks and more large gatherings. “The potential for more frequent—and more intense—exposure over time” plays a role alongside Delta or possible waning vaccine immunity.
“Protection against hospitalization looks quite stable,” Sander says. In the New York study, for example, vaccine efficacy against hospitalization for COVID-19 stayed close to 95%.
The U.S. decision to launch a booster campaign for the general population drew fire as both unnecessary, given that vaccinated people are still largely protected against serious disease, and unethical given the shortage of vaccine doses around the world. But a handful of other rich countries have also taken the step, or plan to. Israel, for example, has begun to roll out booster shots for all people over the age of 50—and is considering expanding the offer to everyone 40 and older. Germany has said it will start to offer boosters to high-risk people next month. In the United Kingdom, some officials have said third doses may begin next month, although no formal decision has been made.
There is broad agreement that for people with weakened immune systems, whether because of age or disease, boosters can offer important protection. “There’s a proportion of the population for whom two shots is not sufficient. For certain groups of people a three-dose regimen is required,” says Sander, who has studied the effects of boosters in immune-compromised patients. He has advised the German government to offer boosters to everyone over age 60. Boosters for health care workers and close contacts of people with weaker immune systems are also likely to be important, he says, to prevent transmission to vulnerable groups and to keep health care workers on the job when hospitals are stretched thin.
But the benefits of boosters for the wider population may not outweigh concerns about vaccine equity, especially for young people at low risk of severe disease, Sander says. Still, he notes, personal choices and the wisest public policy don’t always align: “I’d still rather have a third shot than catch the virus.”
For other vaccines, another dose given months—or years—after initial doses can provide longer lasting protection. A COVID-19 vaccine study in June reported that organ transplant recipients who had responded poorly to two mRNA doses, likely because of the immune suppressant they rely on, responded better to a third dose. (Eight of 24 patients who had no antibodies after two doses developed antibodies after a third dose, and six patients who had low antibody levels all developed high levels after a booster.) Very preliminary data released this week by a healthcare company in Israel suggested a third dose was “86% effective” in preventing infections in people 60 and older, 1 week after receiving a booster—but no other details were provided making the figure almost impossible to assess.
“A third dose is a good idea,” says Akiko Iwasaki, an immunologist at Yale University. Although at-risk groups should receive them first, she adds, “If there are enough doses, I think the general public will benefit.” She notes that the higher viral loads observed in breakthrough infections are independent of age, so even for people in their 20s, a higher level of immunity could help keep the virus in check, helping to prevent transmission.
Aside from severe disease, Iwasaki says, lingering symptoms from mild SARS-CoV-2 infections are a serious public health issue. “We know Long Covid can happen after breakthrough infections. And even if it’s just 1% of infections, enough people are potentially at risk that I think we need to do everything we can to prevent that from happening.”