CDC study shows prior COVID-19 infection and vaccines protected against delta variant

Both vaccination and a prior infection provided protection against infection and hospitalization from COVID-19, but rates were actually lower among people who had recovered compared to people who were vaccinated, according to a new study the Centers for Disease Control and Prevention (CDC) released Wednesday.

The study is likely to further fuel the people who insist natural immunity is just as protective, and won’t get vaccinated because they’ve been infected with COVID-19 at some point and recovered.

The study, however, has a variety of caveats that the CDC was quick to point out. It was conducted in California and New York both before and during the period when delta was the dominant variant, but data ends in November before omicron displaced it.

It was also conducted before most people had received additional or booster COVID-19 vaccine doses. 

It showed people who were unvaccinated and did not have a prior COVID-19 infection remained at the highest risk of infection and hospitalization, while those who were previously infected, both with or without prior vaccination, had the greatest protection. 

“Importantly, 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 waning immune protection, the study found.

In a statement, the CDC emphasized that while both vaccination and previous infection were shown to provide protection, vaccination is the only safe way to ensure lasting protection. 

The agency also noted that viruses are constantly changing, and different variants can have different characteristics. So even though prior infection was shown to be protective during delta, it may not be the case with omicron.  

For example, two previous U.S. studies found more protection from vaccination than from previous infection during periods before delta dominated.

Now, “evidence suggests decreased protection from both vaccine- and infection-induced immunity against Omicron infections, although additional protection with widespread receipt of booster COVID-19 vaccine doses is expected,” the study concluded.

The CDC also noted that the analysis did not include information on the severity of initial infection, and does not reflect the risk of severe disease or death from a COVID-19 infection. By Nov. 30, approximately 130,781 residents of California and New York had died from COVID-19. 

“We know that vaccination remains the safest strategy for protecting against Covid-19,” Benjamin Silk, a CDC epidemiologist, told reporters during a briefing Wednesday.

Case rates were initially lowest among vaccinated persons without a previous COVID-19 diagnosis; however, after emergence of the delta variant and over the course of time, incidence increased sharply in this group, but only slightly among both vaccinated and unvaccinated persons with previously diagnosed COVID-19. 

Infection-derived protection was greater after the highly transmissible delta variant became predominant, coinciding with early declining of vaccine-induced immunity in many people, CDC said.

Similar data accounting for booster doses and as new variants, including omicron, circulate will need to be assessed.