People who are not vaccinated against Covid-19 are two times more likely to be re-infected with the deadly infectious disease, according to a study by the US Center for Disease Control and Prevention (CDC).

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The agency said in a statement, "Being unvaccinated was associated with 2.34 times the odds of re-infection compared with being fully vaccinated." It added, "To reduce their likelihood for future infection, all eligible persons should be offered COVID-19 vaccine, even those with previous SARS-CoV-2 infection."

Although laboratory evidence suggests that antibody responses following COVID-19 vaccination provide better neutralisation of some circulating variants than does natural infection, few real-world epidemiologic studies exist to support the benefit of vaccination for previously infected persons, the CDC said.

The team involved 246 people to understand the association between vaccination and SARS-CoV-2 re-infection in Kentucky during May-June 2021 among persons previously infected with SARS-CoV-2 in 2020. The findings suggest that among persons with previous SARS-CoV-2 infection, full vaccination provides additional protection against re-infection.

Partial vaccination was not significantly associated with re-infection, the study showed. "The lower odds of re-infection among the partially vaccinated group compared with the unvaccinated group is suggestive of a protective effect and consistent with findings from previous studies indicating higher titres after the first mRNA vaccine dose in persons who were previously infected," the CDC said.

However, it "should be interpreted with caution given the small numbers of partially vaccinated persons included in the analysis", the agency added. Further, the study also acknowledges its limitations, such as re-infection was not confirmed through whole genome sequencing, which would be necessary to definitively prove that the re-infection was caused from a distinct virus relative to the first infection; and that people who have been vaccinated are possibly less likely to get tested and thus the link between re-infection and lack of vaccination might be overestimated. The team suggested additional studies with larger populations to support the findings.