Even with new omicron subvariants, the third and fourth doses of the mRNA vaccine remain protective, CDC says

While the effectiveness of mRNA vaccines has declined with the emergence of different omicron subvariants, injection of a third and fourth dose still protects eligible adults over 50 from the risk of hospitalization and critical care, according to the latest US data published by epidemiologists -Centers for Disease Control and Prevention (CDC).

This report is one of the first to provide accurate data on vaccine efficacy against variants BA.2 and BA.2.12.1. On July 9, they represented 1.4% and 17.3%, respectively, of the sequences performed in the United States, while BA.5 became the majority with 65% of the sequences; BA.4 represents 16.3% of the sequences.

In France, the BA.5 is also in the majority (72% according to the Flash poll of June 27th) and has replaced BA.2, the variant BA.2.12.1 is still weakly represented. However, although recent data indicate a similar presence of neutralizing antibodies against BA.1 and BA.2 in vaccinated individuals, it appears that the BA.2.12.1 variant is more able to evade the immune system.

A case-control study

During the period dominated by the BA.2 and BA.2.12.1 variants, the overall vaccine efficacy of mRNA vaccines – the authors excluded patients who received at least one dose of non-mRNA vaccine and those who received at least one dose of non-mRNA vaccine in the past – lower than during those dominated by the BA.1 variant.

CDC data show that a third dose of the vaccine was associated with slightly greater protection in all age groups, and that a fourth dose was associated with an additional gain in efficacy in eligible populations, ie, immunocompetent adults over 50 years of age.

The effectiveness of the vaccine was assessed by members of the Vision research network, who reviewed hospital admissions data from 214,487 emergency and intensive care unit admissions and 58,782 hospital admissions for a variety of reasons, including understanding of Covid. This data was collected between December 2021 and June 2022 in 10 American states. The researchers evaluated this effectiveness by conducting a case-control study in which they estimated the likelihood of vaccination or non-vaccination for people who tested positive or negative for Sars-CoV-2. These results were adjusted for age and epidemic dynamics.

Of the 214,487 patients admitted to the emergency or intensive care unit with a Covid diagnosis, more than half (57.8%) arrived in the period dominated by BA.1. At this time, 32.9% of these patients were Covid positive, 41.4% were unvaccinated, 32.3% had received two doses and 26.3% had received three doses.

Of patients admitted to these services when variants BA.2 and BA.2.12.1 were dominant, 11.3% were positive for Sars-CoV-2, 30.9% were unvaccinated, 25% had two doses received, 39.6% had received three doses and 4.5% had received four doses. In fact, between these two periods, a fourth dose of vaccine is approved for immunocompetent patients over 50, at least four months after the third dose.

In the two periods considered, having received three doses was associated with better vaccine efficacy against the risk of admission to the emergency room and intensive care unit than having received two doses. Furthermore, this vaccine efficacy was greater when BA.1 was dominant than when the BA.2 and BA.2.12.1 variants were dominant.

An efficiency that nevertheless deteriorates

During the period dominated by BA.1, vaccination against Covid reduced the risk of hospitalization by 92% between 7 and 119 days after the third injection and by 85% beyond 120 days. Once BA.2 and BA.2.12.1 became dominant, vaccine efficacy was only 69% during the first 119 days and 52% beyond 120 days. In immunocompetent adults over 50 years of age, the fourth dose increased vaccine protection from 55% to 80%.

“Immunocompetent individuals should receive the recommended booster doses of Covid-19 vaccine to prevent moderate to severe forms of Covid-19 […] these booster doses must be available as soon as the person is eligible.”recommend the authors in their discussion.

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