Meanwhile, the highly transmissible omicron subvariant XBB.1.5 — dubbed “the octopus” by some — is now the dominant SARS-CoV-2 strain in the United States, predicted by the CDC to account for just over 49 percent of cases in the country will make out from last week.
Earlier this month, the According to the WHO, XBB.1.5 is the most transferrable variant to date, and is circulating in dozens of countries. Although a catastrophic wave has not yet erupted in the US, there has been a spike in deaths this month, with an average of 564 people dying of Covid-19 every day since January. 18, compared to an average of 384 around the same time in December.
The new vaccine efficacy study, which used data from the National Pharmacy Program for Covid testing, found that the bivalent booster vaccine provided 48 percent greater protection against symptomatic infections from subvariants XBB and XBB.1.5 in people who received the booster in the previous two to three months, compared to people who had previously received only two to four monovalent doses.
It also offered 52 percent greater protection against symptomatic infection by the BA.5 subvariant, although BA.5 accounted for only about 2 percent of U.S. cases, according to CDC estimates last week.
CDC officials warned that the results reflect a population-level protection rate and that individual risk of infection varies.
“It’s hard to interpret it as an individual risk because everyone is different,” said Ruth Link-Gelles, the author of the vaccine efficacy study published Wednesday in the MMWR. “Their immune system is different, their history of previous infections is different. They may have underlying conditions that put them at greater or lesser risk of contracting COVID-19.”
She also said it’s unclear how long the bivalent booster protection will last, given the limitations of the study.
“It’s too early to know how the bivalent vaccine will go down,” she said. “What we’ve seen in the past is that your protection lasts longer for more serious illnesses. Although your protection against a symptomatic infection may decrease over time, you are likely still protected against a more serious illness for a longer period of time.”