The majority of the vaccines available for COVID-19 have two doses. People are said to become more capable of resisting the virus after the second dose. But, a recent Kaiser Permanente study, found that one month after a third dose, the effectiveness of the Pfizer-BioNTech COVID-19 vaccine was higher for preventing infection and hospitalisation than 2 doses of the vaccine after 1 month.

The study was published in The Lancet Regional Health - Americas. "When we looked at the effectiveness of the 2 doses of Pfizer-BioNTech vaccine versus 3 doses, we see a benefit with 3 doses that exceed that achieved with 2 doses alone," said Sara Y. Tartof, PhD, an epidemiologist with the Kaiser Permanente Southern California Department of Research & Evaluation and a member of the faculty of the Kaiser Permanente Bernard J. Tyson School of Medicine, both in Pasadena.

This study assessed the primary series of two doses of the Pfizer-BioNTech COVID-19 vaccination effectiveness against infection, hospitalisation, and death up to 8 months after vaccination, and also assessed the effectiveness of 3 doses of the vaccine up to 3 months after vaccination.

To assess effectiveness, this research study evaluated electronic health records of 3.1 million members of Kaiser Permanente in Southern California from December 14, 2020, to December 5, 2021. During the study period, 197,535 (6.3 per cent) patients were infected with SARS-CoV-2, and of those, 15,786 (8 per cent) were admitted to the hospital. During the study period, the predominant variant was delta and not omicron.

2-dose vaccine effectiveness against infection declined from 85 per cent during the first month after vaccination to 49 per cent up to 8 months following vaccination. 2-dose vaccine effectiveness against hospitalisation remained high (90 per cent) throughout the 8 months and did not wane, except among people who were 75 years of age and older, or who had compromised immune systems.

For people who were immunocompromised, the protection against hospitalisation dropped to 74 per cent, and for those 75 and older, it was 77 per cent.3-dose vaccine effectiveness was 88 per cent against infection and 97 per cent against hospitalisation within the first 3 months after vaccination.

"What we see from this research is that the public health impact of a third dose to prevent severe disease is substantial," Tartof said. "Importantly, all studies that have evaluated the vaccine effectiveness of a third dose -- including ours -- have shown a meaningful improvement in vaccine effectiveness against a broad range of SARS-CoV-2 outcomes."