A fourth dose of the Pfizer-BioNTech (BNT162b2) vaccine for over 60 has been approved in Israel.

Two retrospective cohort studies now assess its Covid-19 prevention efficacy. A fourth dose (8–14 days prior) was compared to three doses (629,00 recipients) and internal control (days 3–7).

From week four to week six, the four-dose group had 3.5–4.3-fold lower rates of severe disease than the three-dose group and 2.3–2.8-fold lower than the internal control group.

On average, the four-dose group had 2.0–1.1 times lower rates of SARS-CoV-2 infection than the three-dose group and 1.8–1.0 times lower than the internal control.

In a second study, there was a comparison between 182,000 sixty-year-old members of an Israeli healthcare organisation who received a fourth dose to those who received three doses.

Between days 7 and 30 postvaccination, the relative effectiveness of a fourth versus the third dose was 45% against SARS-CoV-2 infection, 55% against symptomatic Covid-19, 68% against Covid-19–related hospitalisation, 62% against severe Covid-19, and 74% against Covid-19–related death.

The studies showed that a fourth dose of mRNA vaccine had a transient effect on infection but a larger effect on severe disease.