The Pfizer-BioNTech Covid vaccine is 90% effective at preventing hospitalisation for up to six months, with no signs of waning during that time period, according to a large new US study conducted by researchers at Pfizer and Kaiser Permanente.
The vaccine also provides powerful protection against the highly contagious Delta variant, the scientists found. In a subset of people who had samples of their virus sequenced, the vaccine was 93% effective against hospitalisation from Delta, compared with 95% against hospitalisation from other variants. “Protection against hospitalisation remains high over time, even when Delta predominates,” said Sara Tartof, an public health researcher at Kaiser Permanente Southern California and the first author of the study.
The vaccine’s effectiveness against infection did decline over time, however, falling from 88% during the first month after vaccination to 47% after five months.
The findings, published Monday in The Lancet, come amid a debate over whether, and when, booster shots may be necessary. Data from Israel indicates that the Pfizer vaccine’s effectiveness against infection dropped to 39% in late June and early July, down from 95% in January through early April. But it remained moover 90% effective against severe disease during that time period.
On the other hand, a recent study from the Centers for Disease Control and Prevention reported that the vaccine’s effectiveness against hospitalization dropped to 77% after four months, compared with 91% in the first few months. In the new study, the researchers found that overall the vaccine was 90% effective against hospitalisation and 73% effective against infection. Among those who were 65 or older, it was 86% effective against hospitalization and 61% effective against infection.