Meanwhile, the highly transmissible Omicron subvariant XBB.1.5 — nicknamed “the Kraken” by some — is now the dominant SARS-CoV-2 strain in the U.S., accounting for just over 49 percent of cases in the U.S., according to the CDC. country will matter since last week.
Earlier this month, the The WHO said XBB.1.5 is the most transmissible variant to date, and circulates in dozens of countries. While no catastrophic wave has yet emerged in the US, there has been a spike in deaths this month, with an average of 564 people dying each day from Covid-19 as of January 18, compared to an average of 384 around the same time in December.
The new vaccine efficacy study, using data from the National Pharmacy Program for Covid Testing, found that the bivalent booster provided 48 percent more protection against symptomatic infection of the XBB and XBB.1.5 subvariants in humans who had the booster in the previous two to three months, compared to people who had previously only received two to four monovalent doses.
It also provided 52 percent more protection against symptomatic infection of the BA.5 subvariant, even though BA.5 accounted for only about 2 percent of US cases last week, according to CDC estimates.
CDC officials cautioned that the findings reflected a population-level level of protection and that individual risk of infection varies.
“It’s hard to interpret it as an individual risk because every individual is different,” said Ruth Link-Gelles, the author of the vaccine effectiveness study published Wednesday in MMWR. “Their immune system is different, their history of previous infection is different. They may have underlying conditions that put them at greater or lesser risk for COVID-19 disease.”
She also said that given the study’s limitations, it was unclear how long the bivalent booster protection will last.
“It’s too early to know how the bivalent vaccine will decline,” she said. “What we’ve seen in the past is that your protection lasts longer with more serious illnesses. So even though you may have less protection against symptomatic infections over time, you’re still likely to be protected against more serious illnesses for a longer period of time.”