According to a large study published in The BMJ, mRNA vaccines are highly effective in preventing COVID hospital admissions related to the alpha, delta, and omicron variants. However, three doses are needed to achieve similar protection against omicron that two doses provide against delta and alpha.
The results also show that, although severity of disease among patients admitted to hospital is lower with the omicron versus delta variant, patients with omicron are still at risk of critical illness and death.
In order to guide vaccination policies and development of new vaccines, it is essential to understand COVID variants and vaccine efficacy.
Early studies suggested reduced vaccine effectiveness against infection and hospital admissions for omicron compared with earlier variants, but little is known about the effectiveness of vaccines to prevent the most severe manifestations of COVID, including respiratory failure and death, for patients with infection due to the omicron variant.
To address this, the researchers assessed COVID severity in the alpha, delta, and omicron variants among hospitalised adults and compared the effectiveness of two and three doses of mRNA vaccines (Pfizer-BioNTech and Moderna) in preventing 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 cases with COVID and 5962 controls without COVID.
Patients were classified into alpha, delta or omicron based on viral gene sequencing or by the predominant circulating variant at the time of hospital admission.
Vaccine effectiveness was then calculated for each variant and variants’ disease severity was compared with the World Health Organization’s clinical progression scale.
Effectiveness of two doses of an mRNA vaccine to prevent COVID hospital admission was found to be lower for the omicron variant than alpha and delta variants (65%, 85%, and 85%, respectively), whereas 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 hospitalised with COVID, 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% of patients admitted to hospital with the omicron variant (vaccinated and unvaccinated) progressing to invasive mechanical ventilation, and 7% dying in hospital.
Nevertheless, vaccinated patients hospitalised with COVID had significantly less sever disease than unvaccinated patients across all variants.
As an observational study, cause cannot be established, and some variant misclassification may have occurred. Changes in clinical management during the periods when the alpha, delta, and omicron variants predominated were not accounted for. These could have affected outcomes, the researchers acknowledged.
Nevertheless, this was a large study with rigorous evaluation of vaccination status and of outcomes beyond hospital admission, suggesting that the results are robust.
As such, they say that mRNA vaccines “were associated with strong protection against hospital admissions with COVID due to the alpha, delta, and omicron variants” and that vaccination against COVID including a third dose of an mRNA vaccine, “is critical for protecting populations against COVID-associated morbidity and mortality.”
They concluded: “As the COVID 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.”
Source: EurekAlert!
