Two doses of the Pfizer-BioNTech vaccine offer highly effective protection against Covid-related hospitalisation for at least six months, a new study has shown.

According to the findings, two doses are 90% effective against hospitalisation for all variants, including the delta variant, for at least six months.

Furthermore, as part of a variant-specific analysis, the researchers found that reductions in the vaccine’s effectiveness against infection over time are most likely due to waning, rather than the delta variant escaping vaccine protection.

The findings are consistent with preliminary research from the US Centers for Disease Control and Prevention (CDC) and the Israel Ministry of Health, which both found reductions against infection after a six-month period.

According to the study’s lead author, Dr Sara Tartof of Kaiser Permanente’s Department of Research and Evaluation in the US, this study confirms that vaccines are “a critical tool for controlling the pandemic and remain highly effective in preventing severe disease and hospitalisation”.

However, she noted that while protection against infection does decline in the months following a second dose, “considerations for booster shots should take global COVID-19 vaccine supply into account, as people in many countries around the world have not yet received a primary vaccination series”.

The study involved an analysis of almost 3.5 million electronic health records between December 2020 and August 2021. During the study period, 5.4% of people were infected with SARS-CoV-2 and among those infected, 6.6% were hospitalised. The average time since being fully vaccinated was three to four months.

Vaccine effectiveness against infections involving the delta variant at one month after two doses was 93%. This fell to 53% after four months.

Effectiveness against other variants at one month after receiving two doses was 97%. This fell to 67% after four months.

However, effectiveness against delta-related hospitalisations remained high (90%) for the duration of the study period.

Details of these findings are published in The Lancet and can be viewed here.