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Third COVID-19 vaccine 'essential' to protect against omicron

Thursday March 10th 2022

Three doses of the mRNA vaccines appear to be necessary to protect people from needing hospital treatment for the omicron COVID-19 variant, researchers report today.

The large US study, published in the latest edition of The BMJ, showed severity of disease among patients admitted to hospital is lower with the omicron variant than the delta variant. But, it also found, patients with omicron are still at risk of critical illness and death.

The findings show two doses of the vaccine provide good protection against the delta and alpha variants – but suggest a third dose is necessary for protection against the worst effects of omicron.

While early studies suggested there was reduced vaccine effectiveness against infection and hospital admissions for omicron compared with earlier variants, little was known about the effectiveness of vaccines to prevent severe COVID-19, including respiratory failure and death, for patients with the omicron variant.

For this observational study, researchers assessed the clinical severity of COVID-19 associated with the alpha, delta, and omicron variants among adults who were admitted to hospital and compared the effectiveness of two and three doses of the mRNA vaccines, Pfizer-BioNTech and Moderna, to prevent hospital admissions related to each variant.

Their findings are based on 11,690 adults who were admitted to 21 hospitals across the USA between March 2021 and January 2022. Of these, 5,728 were COVID-19 cases and 5,962 did not have the virus.

Patients were split into three variant groups, based on viral gene sequencing or by the predominant variant at the time they were admitted to hospital: alpha (11 March to 3 July 2021), delta (4 July to 25 December 2021), and omicron (26 December 2021 to 14 January 2022).

Vaccine effectiveness was calculated for each variant and disease severity was compared among variants using the World Health Organization’s clinical progression scale.

The team found two doses of an mRNA vaccine had reduced effectiveness of preventing hospital admission with COVID-19 with omicron than alpha and delta variants, at 65%, 85%, and 85%, respectively.

However, three doses were 86% effective against the omicron variant.

When they looked at unvaccinated adults admitted to hospital with COVID-19, the delta variant was associated with the most severe disease, followed by alpha and omicron.

Omicron was, however, associated with substantial critical illness and death, with 15% of patients - both vaccinated and unvaccinated - admitted to hospital having to undergo invasive mechanical ventilation, while 7% died in hospital.

Vaccinated COVID-19 patients who needed hospital treatment had significantly lower disease severity than unvaccinated patients for all the variants.

While it was an observational study, the team says it was a large study with rigorous evaluation of vaccination status.

They add that mRNA vaccines were “associated with strong protection against hospital admissions with COVID-19 due to the alpha, delta, and omicron variants” and that vaccination against COVID-19, including a third dose of an mRNA vaccine, “is critical for protecting populations against COVID-19-associated morbidity and mortality”.

“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.”

Lauring AS, Tenforde MW, Chappell JD et al. Clinical severity of, and effectiveness of mRNA vaccines against, COVID-19 associated with omicron, delta, and alpha SARS-CoV-2 variants in the United States: prospective observational study. BMJ 10 March 2022; doi: 10.1136/bmj-2021-069761


Tags: Flu & Viruses | North America | Pharmaceuticals

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