Studies have revealed that an additional dose can provide extended protection against Covid-19. This comes close on the heels of India having started rolling out a precautionary third dose for the general population. Though the protection against infection itself was brief, four doses offered greater protection against Covid-19 than three, according to a study from Israel. The country started giving the fourth dose to people above 60 years from January 2 this year from Pfizer BioNTech which uses the mRNA platform.
The study was conducted between January 10 and March 10 when the Omicron variant was predominant. It has been published in The New England Journal of Medicine. The study was based on Israeli Health Ministry data on 12.5 lakh persons aged 60 or older.
It assessed the rate of confirmed infection and severe Covid starting at 8 days after the fourth dose. The study compared with the rate among persons who had received a fourth dose 3 to 7 days earlier and who had got only three doses. Rates of confirmed SARS-CoV-2 infection and severe Covid-19 found were lower after a fourth dose than after only three doses. Protection against confirmed infection appeared short-lived but protection against severe illness did not wane during the study period.
The study also found the unadjusted number of cases of severe Covid-19 per 100,000 person-days. It was 1.5 in those who got four doses, 3.9 in those who got three doses, and 4.2 in the internal control group which got the fourth dose 3 to 7 days earlier. The rate of severe Covid in the fourth week after the fourth dose was lower than that in the three-dose group by a factor of 3.5 based on adjustments done. It was lower than that in the internal control group by a factor of 2.3.
The adjusted rate was lower than in the three-dose group by a factor of 2.0 and was lower than that in the internal control group by a factor of 1.8. According to the study, protection against severe illness did not wane during 6 weeks after the fourth dose but this protection waned in later weeks.