Even as the United States made the controversial announcement this week that it would begin to offer COVID-19 vaccine booster shots in September, scientists and public officials were scrambling to assess the rationale officials offered: that the vaccines’ protection against the Delta variant of SARS-CoV-2 is waning.
When vaccines were developed and delivered earlier this year, the pandemic coronavirus seemed to almost magically melt away in some countries. But now, as Delta infections surge in highly vaccinated countries that once seemed to have COVID-19 on the run, including Israel and the United Kingdom, many fully vaccinated people are wondering about their level of protection. Although most data still show the vaccines are very effective at preventing severe disease and death, the initial hopes that they could also suppress transmission and completely prevent “breakthrough” infections in vaccinated people are not supported. What is uncertain is how much the trends reflect a possible decline in vaccine-induced immunity versus the extraordinarily infectious nature of the Delta variant, and whether widespread use of boosters is now valid or even warranted.
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 Delta 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.
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.
“It could be that Pfizer’s protection drops from its initially very high levels and then stabilizes, or it could be that people who have had two doses of Pfizer will need a third,” says Sarah Walker, an epidemiologist at Oxford who led the U.K. study. In Israel, which used only the Pfizer vaccines, researchers also found that people fully immunized in January had twice the risk of being infected with SARS-CoV-2 during June and July as people who were vaccinated in April.