Vaccinated With Prior Infection Most Protected During Delta Wave

Following the emergence of the Delta variant, individuals who had a prior COVID-19 infection with or without vaccination had lower case rates than those who were vaccinated alone, data from California and New York found.

"Infection-derived protection was higher after the Delta variant became predominant, a time when vaccine-induced immunity for many persons declined because of immune evasion and immunologic waning," reported Tomàs León, PhD, of the California Department of Public Health, and colleagues, writing in an early edition of the Morbidity and Mortality Weekly Report.

Prior to the Delta variant becoming prevalent, the highest incidence of COVID-19 was in unvaccinated individuals without a prior COVID infection, and vaccination demonstrated the most protection against a new infection. In California and New York, respectively, rates among the vaccinated and those with a prior infection were:

• Vaccinated alone: 19.9-fold and 18.4-fold lower
• Vaccinated with prior infection: 9.6-fold and 8.5-fold lower
• Unvaccinated with prior infection: 7.2-fold and 9.9-fold lower

By early October 2021, however, that balance shifted. Compared to unvaccinated individuals without a prior COVID infection in California and New York, rates of COVID infection were:

• Vaccinated alone: 6.2-fold and 4.7-fold lower
• Vaccinated with prior infection: 32.5-fold and 19.8-fold lower
• Unvaccinated with prior infection: 29.0-fold and 14.7-fold lower

CDC released a statement on the findings, noting important caveats -- namely that the analysis was conducted prior to the spread of Omicron and before widespread implementation of booster doses, and also that the data set did not include information on severity of initial infection among those who were previously infected.

They cited a recent study that found that "with increasing time since prior infection, vaccination provides greater protection against COVID-19."

León's group examined hospitalization, surveillance and immunization registry data from May to November 2021 in California and New York. A little less than three-quarters (71-72%) of the cohort in both states were vaccinated and without a prior COVID infection, about 18% were unvaccinated with no prior infection, about 5% were vaccinated with a prior infection, and 5-6% of individuals were unvaccinated with a prior infection.

California also provided data on hospitalization rates, and found similar patterns. In the pre-Delta period, compared with unvaccinated individuals with no prior infection, hospitalization rates were:

• Vaccinated alone: 27.7-fold lower
• Vaccinated with prior infection: 7.1-fold lower
• Unvaccinated with prior infection: 6.0-fold lower

Yet, by early October 2021, the numbers shifted as well. Compared to unvaccinated individuals with no prior infection, hospitalization rates were:

• Vaccinated alone: 19.8-fold lower
• Vaccinated with prior infection: 57.7-fold lower
• Unvaccinated with prior infection: 53.5-fold lower

When stratified by vaccine received, the highest incidence of COVID cases among those without prior infection were in those who received Johnson & Johnson's vaccine, followed by Pfizer's vaccine and then Moderna's vaccine. However, this was not observed in vaccinated individuals with prior infection, the authors said.

As initial infection among unvaccinated individuals "increases risk for serious illness, hospitalization, long-term sequelae and death," they concluded that "vaccination remains the safest and primary strategy to prevent SARS-CoV-2 infections, associated complications, and onward transmission."

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