In England, everyone in their late teens and 20s became eligible for shots only in mid-June, two months later than in the United States, and many are still waiting for second doses. Those second doses have become all the more crucial as Delta spreads, as the variant overwhelms the first doses in some cases.
In one study published in the journal Nature last week, only about 10 percent of blood samples from people who received one dose of either the AstraZeneca or the Pfizer-BioNtech vaccines were able to neutralize the Delta variant, compared with 95 percent of those who got both doses. (Other studies suggest that a single dose is at least enough to prevent serious illness and death, however.)
More than 90 percent of people older than 55 are fully vaccinated in Britain. That has not entirely blunted the toll on hospitals following the spread of the Delta variant: Patient admissions have begun climbing as quickly as cases in recent days, a reminder that some infections still inevitably lead to severe illness. But the proportion of cases leading to hospitalizations is lower than it was in previous waves.
“The actual transmission pattern is really strongly concentrated in the unvaccinated population, which in the U.K. is almost all young people,” said Jeffrey Barrett, who directs the coronavirus sequencing initiative at the Wellcome Sanger Institute. “You get cases, but they don’t usually get very sick.”
In the United States, some states are already seeing a rise in hospitalizations. Even if those numbers remain small compared with last winter’s, they will strain hospitals in states like Oregon, already at maximum capacity as a result of other factors, like the heat wave.
“We don’t really have a huge margin for error,” said Brian O’Roak, a geneticist at Oregon Health and Science University in Portland. “If we do see a sharp rise in hospitalizations, we’re going to be back where we were during the last surge.”