WASHINGTON, United States -- During America's last surge of the coronavirus driven by the Delta variant, people who were unvaccinated but survived Covid were better protected than those who were vaccinated and not previously infected, a new study said Wednesday.

The finding is the latest to weigh in on a debate on the relative strengths of natural versus vaccine-acquired immunity against SARS-CoV-2, but comes this time with the imprimatur of the US Centers for Disease Control and Prevention (CDC).

The authors of the paper warned, however, against depending on infection as a strategy, given the higher risks to unvaccinated persons of hospitalization, long term impacts, and death, compared to vaccinated people.

"Viruses are constantly changing, including the virus that causes Covid-19," the CDC said in a statement.

"The level of protection offered by vaccination and surviving a previous infection changed during the study period. Vaccination remains the safest strategy for protecting against Covid-19," it added.

The analysis was also carried out before the emergence of the Omicron variant, for which both vaccine and infection-derived immunity appear diminished, and before boosters were made widely available.

The new study involved patients in New York and California between May 30 to November 30, 2021.

Prior to Delta becoming dominant, vaccination conferred greater immunity than infection. But the relationship shifted when the variant became predominant in late June and July.

By the week of October 3, case rates among vaccinated people without prior Covid were around six times lower in California and five times lower in New York compared to those who were unvaccinated and without prior Covid.

But the rates were substantially lower among people with previous Covid, including around 29 times (California) and 15 times lower (New York) among unvaccinated persons with a previous diagnosis, compared to those who were unvaccinated and without prior Covid.

Protection was highest among those who had both vaccination and prior Covid.

Hospitalizations followed a similar pattern.

Other research, including a notable paper from Israel in August, have similarly found that natural immunity was more potent than vaccines during the Delta surge.

But the US CDC had previously taken the opposite position, based on pre-Delta data.

"Further studies are needed to establish duration of protection from previous infection by variant type, severity, and symptomatology, including for the Omicron variant," the paper concluded.