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(WJW) – Natural immunity to COVID was more effective against the delta variant than vaccines alone, according to a recent CDC study.

The study looked at COVID-19 cases and hospitalizations in California and New York between May and November of 2021, determining that numbers were highest among unvaccinated people who hadn’t been previously diagnosed with the virus.

Prior to delta becoming the predominant variant in June, cases were higher among people who had COVID before than those who were vaccinated alone, the study said.

During the week of May 30, compared with case rates of unvaccinated people without a previous diagnosis, the study said case rates were 19.9 times lower for vaccinated people without a previous diagnosis in California (18.4 times lower in New York).

Case rates were 7.2 times lower for unvaccinated Californians with a previous diagnosis (9.9 times lower in New York) and 9.6 times lower for vaccinated Californians with a previous diagnosis (8.5 times lower in New York), the study said.

According to the study, however, things flipped when delta came around. By October, people who had natural immunity had lower case rates than those who were vaccinated alone.

By the week of Oct. 3, compared with case rates of unvaccinated people without a previous diagnosis, the study said case rates were 6.2 times lower for vaccinated people without a previous diagnosis in California (4.5 times lower in New York).

At the same time, the study said case rates were 29 times lower for unvaccinated Californians with a previous diagnosis (14.7 times lower in New York) and 32.5 times lower for vaccinated Californians with a previous diagnosis (19.8 times lower in New York).

The study suggests that waning vaccine-induced immunity caused this change since it was conducted before most people got booster shots.

The study also doesn’t factor in the recent surge from the omicron variant, which came later.

The CDC still recommends COVID-19 vaccines and additional booster shots as the best protection against the coronavirus.