Three doses of a vaccine are needed to protect against the fast-spreading Omicron variant of the coronavirus, research in the US found.
The study, published in the British Medical Journal, findings that the Omicron variant causes less severe disease than previous strains, but is more resistant to mRNA vaccines.
Patients who contract the variant are still at risk of death or serious illness.
The peer reviewed research from hospitals across the US sought to assess the effectiveness of vaccines in preventing the most severe cases of Covid-19.
Researchers looked at the clinical severity of Covid-19 associated with the Alpha, Delta, and Omicron variants among adults admitted to hospital and compared the effectiveness of two and three doses of mRNA vaccines Pfizer-BioNTech and Moderna to prevent hospital admissions.
Patients were classified into three variant groups based on viral gene sequencing or by the predominant circulating variant at the time of hospital admission: Alpha (March 11 to July 3, 2021), Delta (July 4 to December 25, 2021), and Omicron (December 26, 2021 to January 14, 2022).
Effectiveness of two doses of an mRNA vaccine to prevent hospital admission with Covid-19 was found to be lower for the Omicron variant (65 per cent) than Alpha and Delta variants (both 85 per cent).
However, three doses were found to achieve 86 per cent effectiveness against the Omicron variant, similar to two doses against the Alpha and Delta variants.
Among unvaccinated adults admitted to hospital with Covid-19, the Delta variant was associated with the most severe disease, followed by the Alpha variant and then the Omicron variant.
The Omicron variant was, however, associated with substantial critical illness and death, with 15 per cent of patients admitted to hospital with the Omicron variant (vaccinated and unvaccinated) progressing to invasive mechanical ventilation, and 7 per cent dying in hospital.
Vaccinated patients admitted to hospital with Covid-19 had significantly lower disease severity than unvaccinated patients for all the variants.
“As the Covid-19 pandemic continues to evolve, routine monitoring of vaccine effectiveness, especially against severe disease, and surveillance programmes to identify viral variants will be essential to inform decisions about booster vaccine policies and vaccine strain updates.”