mRNA vaccines are highly effective in preventing COVID-19 associated hospital admissions related to the alpha, delta, and omicron variants.
In a new study, researchers found three doses are needed to achieve a similar level of protection against omicron that two doses provide against the delta and alpha variants.
They also found that although the severity of disease among patients admitted to the hospital is lower with the omicron versus delta variant, patients with omicron are still at risk of critical illness and death.
Early studies suggested reduced vaccine effectiveness against infection and hospital admissions for omicron compared with earlier variants.
In the study, the team assessed the clinical severity of COVID-19 associated with the alpha, delta, and omicron variants among adults admitted to the hospital.
They compared the effectiveness of two and three doses of mRNA vaccines (Pfizer-BioNTech and Moderna) to prevent hospital admissions related to each variant.
Their findings are based on 11,690 adults admitted to 21 hospitals across the United States between March 2021 and January 2022: 5728 with COVID-19 (cases) and 5962 without COVID-19 (controls).
Patients were classified into three variant groups: alpha (11 March to 3 July 2021), delta (4 July to 25 December 2021), and omicron (26 December 2021 to 14 January 2022).
The team found the effectiveness of two doses of an mRNA vaccine to prevent hospital admission with COVID-19 was lower for the omicron variant than alpha and delta variants (65%, 85%, and 85%, respectively).
But three doses were found to achieve 86% 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, linked to substantial critical illness and death, with 15% of patients admitted to hospital with the omicron variant (vaccinated and unvaccinated) progressing to invasive mechanical ventilation, and 7% dying in hospital.
Vaccinated patients admitted to the hospital with COVID-19 had much lower disease severity than unvaccinated patients for all the variants.
The team says that mRNA vaccines were associated with strong protection against hospital admissions with COVID-19 due to the alpha, delta, and omicron variants.
Vaccination against COVID-19, including a third dose of an mRNA vaccine, is critical for protecting populations against COVID-19-associated morbidity and mortality.
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The study is published in The BMJ and was conducted by Adam S Lauring et al.
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