Benefits from a fourth mRNA dose against COVID are now supported by the latest research from Israel, which in January began administering a fourth dose of Pfizer to people over 60 and others considered high risk.
The study, Protection by a Fourth Dose of BNT162b2 against Omicron in Israel, provides evidence for the short-term effectiveness of a fourth vaccine dose in reducing rates of infection as well as the severity of illness caused by the Omicron variant for this category, as compared with a third dose administered more than four months earlier.
Confirmed infection rates were twice as low among high-risk individuals given a fourth dose for up to a month, though after this time the fourth shot’s effectiveness in preventing infection began to wane, almost disappearing after eight weeks.
The rate of severe illness was also reduced by a factor of 3.5 for four weeks after receiving a fourth dose,1 even though the Omicron variant is genetically divergent from the ancestral SARS-CoV-2 strain for which the Pfizer vaccine was tailored.
Severe illness continued to occur at lower rates in the four-dose groups in later weeks and no signs of waning were evident by the sixth week – though the study period was not long enough to determine exactly how long this protection lasts.
The research, published on April 5 in the New England Journal of Medicine, concluded that “rates of confirmed infection and severe COVID were lower after a fourth dose of vaccine than after only three doses. Protection against confirmed infection appeared short-lived, whereas protection against severe illness did not wane during the study period.”
Their findings have been supported by another Israeli study (not yet peer-reviewed) from Assuta Ashdod University Hospital, which found that a fourth shot halves the risk of mortality among severely ill patients over 60 and halves the odds that a patient will need ventilation.
The research was conducted by a team from the Israel Health Ministry, the Weizmann Institute of Science, the Israel Institute of Technology, the Hebrew University of Jerusalem, Tel Aviv University and the Sheba Medical Centre, using information from the Israeli Ministry of Health national database to examine the success of another booster.
More than 1.25 million older Israelis were monitored between January 10 and March 2, during Israel’s fifth wave of COVID, following the emergence of the Omicron variant in late December 2021.
By January, the prevalence of confirmed infection in Israel had risen sharply, peaking later that month before receding, though by the middle of February (and the study period) there were still more than 20,000 new COVID cases reported each day.
[1] Bar-On et al. (2022). Protection by a Fourth Dose of BNT162b2 against Omicron in Israel. The New England Journal of Medicine. DOI: 10.1056/NEJMoa2201570