Covid-19 bivalent boosters' protection against death and hospitalization in elderly people began waning as soon as two months after vaccination, according to a preprint study.

The findings build on previous reports about the effectiveness of updated boosters from Moderna and Pfizer, which showed the shots sharply reduced risks of severe Covid in older adults, but didn't assess how long protection lasted. The new report, written by researchers who looked at data from a Finnish patient registry, has not yet been peer-reviewed and was published as a preprint in the server MedRxiv.

"Because we found signs of waning already after 60 days since bivalent vaccination, additional boosters for the elderly could be an option at some time point in the future," the authors wrote. "However, the need for further boosting should also be considered in the light of the epidemic situation and economic analyses."

The study looked at hospitalizations and deaths among boosted and non-boosted adults, all of whom had completed the initial two-dose vaccine series. Nearly 1.2 million elderly patients and 444,683 chronically-ill individuals were included. The study period lasted from September 2022, when the updated boosters were first rolled out, to the end of January 2023. During that time period, several omicron variants were circulating.

The omicron-targeted shots also did not reduce the risk of severe Covid in chronically ill adults younger than 65, the study found. Only a small proportion of the group - 15% - received an updated booster, which the authors said may have affected the results. In comparison, 52% of the elderly group received an updated booster.

The European Medicines Agency authorized bivalent boosters targeting the BA.1 variant, as well as the BA.4 and BA.5 boosters used in the U.S. About a third of vaccinated people in the study received the BA.1 booster and the remainder got the BA.4/5 shot. Among the elderly cohort, similar reductions in severe Covid were observed with both versions of the updated boosters, according to the study.

Another study performed by the U.K. Health Security Agency found similar effectiveness with the BA.1 booster, but results differed from the Finnish study on how long protection lasted. In the U.K. study, the boosters continued to confer high levels of protection for at least 10 weeks after vaccination.