23 May 2024

A shot of an mRNA vaccine as a booster affords some level of protection against Omicron infection in the short term. The duration of this protection and its effectiveness against severe disease are, however, uncertain, according to medical research conducted in Ontario, Canada recently and published on MedRxiv Preprint server.

The researchers collected data from over 470,000 subjects, including patients infected with Omicron, Delta and non-infected people between November 22nd and December 19th, 2021. The objective of the study was to estimate vaccine effectiveness (VE) against infections caused by Omicron or Delta in Ontario.

The results demonstrate that “the effectiveness of 2 doses of COVID-19 vaccines against infection (irrespective of symptoms or severity) is substantially lower for Omicron than Delta, and that VE against Omicron infection was only 37% ≥7 days following a third dose.” The scientists also observed negative VE against Omicron among those who had received 2 doses, compared to unvaccinated individuals.

The data shows that the protection level of 2 doses against the Omicron variant, one of which was an mRNA vaccine, is much lower than protection against Delta. After the mRNA booster shot, however, the data shows a higher level of protection against the Omicron variant 7 days after receiving the shot.

The report said there are some limitations to their analyses, such as sample size constraints and the lag between infection and hospitalisation or death.

“Our findings have potentially important implications for proof of vaccination requirements,” the report stated.

In conclusion, the report said, two doses of COVID-19 vaccines are unlikely to protect against Omicron infection. While VE against Omicron infection is substantially lower than against Delta infection, a third dose of an mRNA vaccine affords some level of protection against Omicron infection in the short term. The duration of this protection and effectiveness against severe disease are, however, uncertain. Additional tools beyond the currently available vaccines, such as public health measures, antivirals and updated vaccines, are likely to be needed to protect against Omicron infection.