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— KittyPooh (@KittyPo80176717) July 18, 2022

Effectiveness of a fourth dose of covid-19 mRNA vaccine against the omicron variant among long term care residents…. | ⁦@bmj_latest⁩ A 4th dose increased protection against infection, symptomatic infection, and severe outcomes during Omicron. https://t.co/7HqnTnYwnC

— Carlos del Rio (@CarlosdelRio7) July 12, 2022

A 4th dose in frailer elderly gives extra protection, but mainly against severe outcomes.
The marginal effectiveness of a fourth dose versus a third dose was 19% against infection, 31% against symptomatic infection, and 40% against severe outcomes. https://t.co/qqLMfCdQpQ

— Peter Collignon (@CollignonPeter) July 12, 2022