Antibody levels following COVID-19 vaccination decrease more rapidly than previously believed, according to a study of staff at a hospital in Stockholm, Swedish Television reported on Tuesday.

Seven months after receiving the second dose, the number of antibodies had decreased by 85 percent among those immunized with the Pfizer vaccine — where staff had not been infected with the virus before vaccination.

However, the decrease was considerably slower among those who had been infected with the virus before receiving the vaccine.

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Among those vaccinated with AstraZeneca, researchers observed an even greater decrease.

“As expected, antibody levels fall over time, but I am surprised that they have dropped so significantly in such a relatively healthy and young group,” research leader Charlotte Thalin told Swedish Television.

“The low levels mean that we can have a widespread of infection even in vaccinated groups, which may have consequences for our elderly. We must, therefore, give them a third dose as soon as we can,” she added.

More than 2,000 hospital employees were observed for the study, and the latest findings were based on a subset of more than 460 staff, Swedish Television reported.

find the study at thelancet.com