New data released Wednesday showed that both vaccination and prior infection offered strong protection against infection and hospitalization from Covid-19 during the Delta wave — and that case and hospitalization rates were actually lower among people who had recovered from Covid-19 than among those who had been vaccinated.
The data, released by the Centers for Disease Control and Prevention and health agencies in California and New York, are sure to inflame arguments from those who insist they don’t need to be vaccinated if they can show they’ve recovered from Covid-19. But the data contain many caveats that health officials stressed pointed to the value of vaccination, even on top of prior infection.
For one, the new report was based on data only through November, before the U.S. booster campaign really took off. It also looked at data during the Delta wave and does not account for the surging Omicron variant.
And while research has shown that infection can train the immune system to guard against the coronavirus in different ways than vaccination, Covid-19 also has killed more than 850,000 people in this country, sickened — often severely — millions more, and caused untold cases of long Covid. Serious side effects from the Covid-19 vaccines are extremely rare.
“We know that vaccination remains the safest strategy for protecting against Covid-19,” Benjamin Silk, a CDC epidemiologist, told reporters Wednesday.
The data also confirmed something we’ve known for a long time: Those who weren’t vaccinated and also hadn’t been previously infected were far more likely to be infected and hospitalized than either group.
The new report examined Covid-19 trends among adults in New York and California from May 30 to Nov. 20, 2021.
In early October, after Delta became dominant, infection rates among vaccinated people who hadn’t had Covid were 6.2-fold lower than among unvaccinated people who hadn’t had Covid-19 in California, and 4.5-fold lower in New York. People who previously had Covid-19 but had not been vaccinated had 29-fold (California) and 14.7-fold (New York) lower case rates. Vaccinated people who had also had Covid-19 had the lowest rates, with a 32.5-fold (California) and 19.8-fold (New York) lower infection rate than people who had no protection.
Hospitalization rates in California followed a similar pattern, the report says. (There were no hospitalization data from New York.) In October, hospitalization rates for people who’d been vaccinated but hadn’t had Covid were 19.8-fold lower than among those who hadn’t had Covid-19 or been vaccinated. The rates were 55.3-fold lower among unvaccinated people who’d had Covid-19, and 57.5-fold lower among people who’d been vaccinated and had Covid-19.
Erica Pan, California’s state epidemiologist, said hospitalizations among those who were vaccinated were mostly among older people.
Incidences among people who’d been vaccinated were highest among people who received the Johnson & Johnson shot, followed by the Pfizer-BioNTech and then the Moderna shots, the report said.
“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 immunologic waning,” the report states. Immune evasion refers to how, as the virus evolved, it started to erode the protection elicited by vaccination or an infection from an earlier form of the virus; this happened to some degree with the Delta variant, and to a much larger extent with the Omicron variant.
The new CDC report notes that the analyzed data are from the period before most people had received additional shots. It was only in mid-October, for example, that the government authorized booster shots for people who had received the J&J vaccine, recommending that people get them two months after the original jab of the one-dose shot. Boosters weren’t given the green light for all adults until November.
Boosters not only ramp up the levels of protection that can wane over time after the primary series, studies have shown, but can also deepen the immune response in a way that allows people to better respond to different variants.
The report also notes that the data might not be as relevant during the Omicron period.
Studies have shown that while prior infection seems to generally protect otherwise healthy people from severe disease in the face of Omicron, it does not offer much of a shield against infection. For vaccinated people, boosters restore some of the protection that’s been lost in the face of Omicron, but studies have also shown that some of the most powerful protection is generated when people who’ve had Covid are also vaccinated.
The new study underscores that both infection and vaccination offer protection, said Eli Rosenberg of New York State Department of Health. But, he added, “having Covid the first time carries with it significant risks.”
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