Vaccination 'remains the safest strategy' to protect against COVID-19
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A study conducted in two U.S. states between May and November last year found that both vaccination and prior infection protected people against infection and hospitalization from COVID-19, researchers reported in MMWR.
Hospitalization rates were much lower in these two groups compared with unvaccinated people throughout the study period, with case rates initially lowest among vaccinated people, according to Tomás M. León, PhD, a statistician and infectious disease modeler at the California Department of Public Health, and colleagues.
During the wave of infections caused by the delta variant, however, the researchers found that people who had survived a previous infection had lower case rates than people who were vaccinated but never infected. León and colleagues noted that this shift coincided “with the early declining of vaccine-induced immunity in many persons.”
“The data are not surprising to me,” Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, told Healio. “Prior infection provided significant protection against severe disease in the pre-omicron era, as did vaccination. Hybrid immunity, however, does appear to the most protective and that is why those with prior infection should receive at least one dose of a COVID-19 vaccine — and the CDC should consider those individuals ‘up-to-date’ with vaccination.”
Caveats
In a statement accompanying the study, the CDC noted a few caveats, including that the study period came before the widespread implementation of booster doses — which the agency began recommending for all adults on Nov. 30 — and that the findings could not be applied to the current wave of infections caused by the omicron variant.
“Omicron has changed the dynamics, and the findings may not be fully applicable to omicron, which appears able to evade immunity from prior infection and vaccines almost at will,” Adalja said.
The CDC also noted that the analysis “did not include information on the severity of initial infection and does not reflect the risk of morbidity and mortality from COVID-19 infection.”
“Vaccination remains the safest strategy for protecting against COVID-19,” the CDC said.
Data from California and New York
The data were from California and New York. To assess the impact of previous infection and vaccination on COVID-19 case and hospitalization rates in the two states, León and colleagues collected testing, surveillance and COVID-19 immunization data from four cohorts unvaccinated adults with no previous laboratory-confirmed COVID-19 diagnosis, vaccinated adults with no previous COVID-19 diagnosis, unvaccinated adults with a previous COVID-19 diagnosis, and vaccinated adults with a previous COVID-19 diagnosis from May 30 through Nov. 20, 2021.
Overall, the study demonstrated that COVID-19 incidence in both states was highest among unvaccinated persons without a previous COVID-19 diagnosis compared with that among the other three groups during the study period.
During the week of May 30, COVID-19 case rates in California and New York, respectively, were 19.9-fold and 18.4-fold lower among vaccinated people without a previous COVID-19 diagnosis, 7.2-fold and 9.9-fold lower among unvaccinated people with a previous diagnosis, and 9.6-fold and 8.5-fold lower among vaccinated people with a previous diagnosis, compared with case rates among unvaccinated people without a previous diagnosis, with hospitalization rates following a similar pattern.
By the week of Oct. 3, case rates among vaccinated people without a previous COVID-19 diagnosis were 6.2-fold and 4.5-fold lower in California and New York, respectively, compared with unvaccinated people without a previous COVID-19 diagnosis. These rates were “substantially lower” among both groups with previous COVID-19 diagnoses, including 29-fold and 14.7-fold lower among unvaccinated persons with a previous diagnosis, and 32.5-fold and 19.8-fold lower among vaccinated persons with a previous diagnosis of COVID-19 in California and New York, respectively. Hospitalization rates, again, followed a similar pattern.
Over the next 2 weeks, hospitalization rates among vaccinated people who never had COVID-19 were 19.8-fold lower compared with unvaccinated people who never had COVID-19. But they were even lower — 55.3-fold lower — among unvaccinated people with a previous case of COVID-19. They were lower still — 57.5-fold lower — among those who were vaccinated and had a previous infection.
The CDC said it will publish data later this week on vaccines and boosters during the omicron wave and pointed out a study published recently in Clinical Infectious Diseases demonstrating that, with increasing time since infection, vaccination provides greater protection than prior infection alone.
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