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Omicron is a dangerous variant and having been infected with the Delta variant once does not guarantee that he will not be reinfected with Omicron. Vaccine effectiveness against Omicron is only 37% for the third dose. Two doses of COVID-19 vaccines are unlikely to protect against infection by Omicron.
Results We included 3,442 Omicron-positive cases, 9,201 Delta-positive cases, and 471,545 test-negative controls. After 2 doses of COVID-19 vaccine, vaccine effectiveness against Delta infection declined steadily over time but recovered to 93% (95%CI, 92-94%) ≥7 days after receiving an mRNA vaccine for the third dose. In contrast, receipt of 2 doses of COVID-19 vaccines was not protective against Omicron. Vaccine effectiveness against Omicron was 37% (95%CI, 19-50%) ≥7 days after receiving an mRNA vaccine for the third dose.
Conclusions Two doses of COVID-19 vaccines are unlikely to protect against infection by Omicron. A third dose provides some protection in the immediate term, but substantially less than against Delta. Our results may be confounded by behaviours that we were unable to account for in our analyses. Further research is needed to examine protection against severe outcomes.
https://www.medrxiv.org/content/10.1....30.21268565v1
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