But the very best immune protection against a COVID-19 infection against Delta came in the form of what's called hybrid immunity, a blend of prior infection and vaccination. That was true regardless of which vaccine a person had received.
It's important when looking at this data to remember that it was gathered before many vaccinated people had booster shots, which early data has suggested can improve the body's memory immune response. And it's unclear what it means at this juncture in the pandemic, when there are so many more cases of the highly infectious Omicron variant, a dramatically different version of the virus than Delta.
The vertical axis here measures the estimated hazard rate — in other words, it's showing a person's risk of getting COVID-19. This rate waxed and waned across the six-month study period as cases climbed and fell.
The top three lines show that vaccinated people in New York without any prior COVID-19 infections (broken down by vaccine brand) had a higher hazard rate than those who had been previously infected. In this group, Moderna recipients (orange) had the best protection, then Pfizer (gray), then J&J (dark blue).
The bottom three lines show that vaccinated people in New York with previous COVID-19 infections (again, broken down by vaccine brand) had a lower hazard rate. In this group, Moderna (light blue) again provided the best protection, followed by Pfizer (light green), then J&J (yellow).
It's possible that the reason Moderna's vaccine provided slightly stronger protection than the other two vaccines did is that it contains a higher dosage of mRNA than Pfizer's vaccine (100 micrograms compared with 30 micrograms per dose). It is also two doses, compared with J&J's single shot.
Moderna's two doses are also spaced one week further apart than Pfizer's, potentially providing a little more time for the body to mount a more robust immune response.
Below, you can see that in California, much like in New York, vaccinated and previously infected people had better immune protection against Delta overall than vaccinated people who'd never contracted the virus. Once again, in both categories, Moderna's vaccine provided the best protection.
This study didn't measure the severity of the COVID-19 cases that people contracted. ER doctors have said that by and large the patients they are seeing with severe, life-threatening cases are unvaccinated.
Given the life-threatening complications that COVID-19 infections can spur — including long COVID, MIS-C, and serious heart issues — the CDC continues to stress that vaccination is the wisest, safest way to deliberately develop immune protection against this virus.
"COVID vaccination helps protect by creating an antibody response without a person having to experience severe illness and death," Benjamin Silk, a CDC epidemiologist, told reporters on Wednesday shortly before this new report was released.
"Vaccines continue to reduce a person's risk of contracting the virus that causes COVID-19 and are highly effective at preventing severe illness. And that's why CDC recommends that everyone remain up to date on their COVID vaccination, regardless of previous infections."
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