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DENVER (KDVR) — Natural immunity was six times stronger during the delta wave than vaccination, according to a new report from the U.S. Centers for Disease Control and Prevention.

The report, published Jan. 19, analyzed COVID outcome data from New York and California, which make up about one in six of the nation’s total COVID deaths.

The study has limits — namely, it was done before the omicron wave and doesn’t factor in any information about boosters. It does, however, broadly agree with studies from other countries.

Vaccines were more effective at preventing infection or serious illness than natural immunity from prior infections before the delta variant became the dominant strain. After delta became the main strain, vaccines alone grew weaker against the virus and natural immunity got much stronger. This could be due in large part to the fact that vaccines began wearing off around the time delta spread, according to the study.

“Importantly, infection-derived protection was greater after the highly transmissible Delta variant became predominant,” reads the report, “coinciding with early declining of vaccine-induced immunity in many persons.”

In California during the week beginning May 30, 2021, COVID-19 case rates were 19.9 times lower among vaccinated people without a previous case, 7.2 times lower among unvaccinated people with a previous COVID-19 case, and 9.6 times lower among vaccinated people with a previous COVID-19 case than they were among unvaccinated people without any previous infection.

During a time when the delta variant was the dominant strain on Oct. 3, it changed. Case rates in vaccinated people without a previous COVID-19 diagnosis were only 6.2 times lower than unvaxxed, uninfected people — three times less effective than pre-delta. They grew to 29 times lower among unvaccinated people with a previous diagnosis, and 32.5-fold among vaccinated people with a previous diagnosis of COVID-19.

Similar figures came from New York.

The study also tracks the same trend with hospitalizations. In California, the jump in protection against serious illness was even more dramatic. It shows the protection afforded by previous infections went up nine times in the delta wave.

The effectiveness of natural immunity with omicron, however, is not fully understood yet. The CDC warns both vaccines and natural immunity have declined with the new dominant variant.