This is an informative overview of what we know; some excerpts:
“[Vaccine] efficacy drops with Delta. That is indisputable,” says Leif Erik Sander, an infectious disease expert at the Charité University Hospital in Berlin. But exactly how much it drops differs across studies. In a report this week analyzing weekly reports on nursing home residents across the United States, researchers found that the messenger RNA (mRNA) vaccines made by Pfizer and Moderna had an efficacy against all infections that went from 75% pre-Delta to 53% after it took over. (The variant accounts for more than 90% of U.S. cases now.)
A large study from the United Kingdom, posted as a preprint yesterday, used the Office for National Statistics COVID-19 Infection Survey, which regularly tests more than 300,000 randomly selected people across the United Kingdom. The study compared the numbers of fully vaccinated and unvaccinated survey participants who tested positive for SARS-CoV-2 during two time periods: December 2020 until 16 May, when the Alpha variant dominated, and 17 May to 1 August, when Delta was dominant. The researchers found that for the two main vaccines in use in the United Kingdom—the Pfizer mRNA vaccine and the adenovirus-based shot developed by the University of Oxford and AstraZeneca—protection against symptomatic infection decreased significantly for the Delta period, to 84% for Pfizer and 71% for AstraZeneca. They also found, consistent with other studies, that compared with breakthrough cases due to the Alpha variant, people with Delta breakthroughs had, on average, much higher viral loads in the nose or throat, suggesting they are more likely to spread the virus to others.
A large study of patient health records in New York released this week told a similar story: The efficacy of the three U.S.-authorized vaccines against all SARS-CoV-2 infections dropped from 91.7% to 79.8% between May and July, as Delta took over in the region.
So, is immunity waning?
Although there is still some debate, lab tests suggest the Delta variant is not particularly good at evading the antibodies produced by vaccines or previous infection. That leaves two more probable explanations for the rise in breakthrough cases: Delta’s ferocious infectiousness or a gradual waning in vaccine-induced immunity. The U.S. nursing home residents who were studied are older and frail, and their response to the vaccine might drop faster than other populations. They were also among the first to get the vaccine—some back in December 2020.
The U.K. study attempted to resolve this issue by focusing on the time period after Delta became dominant and comparing the infection rate with the time since a person received their second vaccine dose. The research team found that breakthroughs did increase slightly with more time. People who received the Oxford-AstraZeneca vaccine had 68% protection against infection 2 weeks after their second shot, and 61% after 90 days. The drop-off was sharper in those who received the Pfizer mRNA vaccine: Fourteen days after the second dose, it seemed to provide 85% protection against all Delta infections, symptomatic or not, but that fell to 75% after 90 days....
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.
Do vaccines still protect against severe disease?
Here the latest data are more reassuring. “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%. Data from the Israeli Ministry of Health suggests protection against severe disease is still nearly 92% for people 50 and younger and 85% for those older than 50. Public Health England estimates that two doses of vaccine provide 96% protection against hospitalization.
