A study reveals that being “out of date” on COVID-19 immunization is associated with a decreased risk of infection.

The Centers for Disease Control and Prevention (CDC) in the United States modified their recommendations for coronavirus disease 2019 (COVID-19) immunization in April 2023. Individuals above the age of six were deemed “up-to-date” with COVID-19 immunization if they had received at least one dose of a COVID-19 bivalent vaccine. Individuals were not deemed “up-to-date” if they had not had a single dose of a COVID-19 bivalent vaccination.

A recent study has not been able to demonstrate the efficiency of the bivalent vaccination, despite the fact that SARS-CoV-2 XBB variants were the prevalent circulating strains. Given that these viral variations continue to be the prevalent circulating strains, it is legitimate to wonder if “up-to-date” persons are protected against COVID-19 when compared to their not “up-to-date” counterparts.

To address this issue, new research published on the medRxiv* preprint service explores whether those who were not “up-to-date” had a higher risk of COVID-19 than people who were “up-to-date.”

Concerning the research
The current investigation was carried out at the Cleveland Clinic Health System (CCHS). On September 12, 2022, the COVID-19 bivalent messenger ribonucleic acid (mRNA) vaccination was made available to employees for the first time. The investigation began on January 23, 2023, when XBB lineages first became the prevalent circulating strains in Ohio.

Participants in the study were CCHS employees at any location on September 12, 2022, and they continued to work after the XBB lineages were dominant. Individuals were eliminated if age and gender information were lacking.

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The time to COVID-19, defined as a positive SARS-CoV-2 nucleic acid amplification test (NAAT), was the outcome variable. The research participants were rigorously tracked until May 10, 2023, allowing for the examination of results up to 100 days after the trial began.

Important discoveries
A total of 48,344 individuals were considered, with 1,445 of them filtered due to job termination. By the end of the trial, 12,841 people in the study group were “up-to-date” on COVID-19 vaccination.

11,187 of these people received the Pfizer vaccination, whereas 1,654 received the Moderna vaccine. During the 100-day research period, 1,475 personnel were infected with SARS-CoV-2.

With a mean age of 43 years, the population was relatively youthful. 46% had a history of COVID-19, and 34% were infected with the Omicron form. Furthermore, 87% of the research population got at least one vaccine dose, and 92% were infected with or vaccinated against SARS-CoV-2.

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COVID-19 risk was lower in the “not up-to-date” group than in the “up-to-date” group. When tertiles of a tendency to get tested for SARS-CoV-2 infection were examined, the group that was not “up-to-date” was not more likely to catch COVID-19.

By taking past infection status into account, the categorization for COVID-19 risk was more correctly supplied. Individuals who were least impacted by the Omicron BQ or BA.4/BA.5 polymorphisms had a considerably decreased incidence of COVID-19. When stratified by the most recent infection date, there was no discernible difference between “up-to-date” and “not up-to-date” people.

One reason that being “up-to-date,” according to the CDC definition, was not associated with a decreased risk of COVID-19 was that the bivalent vaccination was less efficient against the Omicron variant’s XBB lineages. Another reason might be because the CDC definition ignores the protective impact of past infection immunity.

According to the current study, not being “up-to-date” on immunization was related to a reduced incidence of COVID-19 than being “up-to-date.” These findings highlight the difficulties in assessing vaccination protection when efficacy wanes over time and the technique of risk classification is solely based on the receipt of a vaccine with uncertain efficacy.

The study’s main merits are its huge sample size and the fact that it was done in a country that spent substantial resources to effectively track the pandemic’s course. Furthermore, by considering vaccination status as a time-dependent covariate, vaccine efficacy might be determined in real-time.

The current study looked at all infections that were found and made no distinction between asymptomatic and symptomatic infections. Several asymptomatic and moderately symptomatic illnesses might have been overlooked inadvertently, leaving information on previous COVID-19 inadequate.

Furthermore, due to the rarity of serious diseases, the question of whether being “up-to-date” reduced the severity of illness could not be examined. Finally, due to the young age of the research cohort, the effects on immunocompromised people could not be investigated.

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