Second COVID-19 booster shot increases protection against severe illness with some Omicron variants: U.S. study – CTV

July 18, 2022

A study looking at the effectiveness of fourth doses of COVID-19 vaccines, run by the U.S. Centers for Disease Control and Prevention (CDC), has found that a second booster shot of an mRNA vaccine has significant effectiveness against severe illness with some Omicron variants.

The study looked at more than 200,000 emergency department visits and 58,000 hospitalizations with COVID-19 to assess effectiveness against the variants Omicron BA.1 and BA.2/BA.2.12.1. The newer strains of BA.4 and BA.5, which are thought to be driving the current wave of COVID-19 cases in Canada and other regions, were not part of the study.

After the first booster shot, vaccine effectiveness against severe illness with Omicron variants BA.1 and BA.2/BA.2.12.1 was found to be just 68 per cent in the first six months, declining to 52 per cent after that point, according to the CDC.

But after a second booster shot, the CDC found the effectiveness against severe illness with these variants increased to 80 per cent during the first six months. Data after the first six months of a fourth dose isn’t available yet.

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