A new study, published Wednesday by a US health authority, shows that unvaccinated people who have previously contracted COVID-19 are better protected against the delta variant than those vaccinated alone.
However, the Centers for Disease Control and Prevention (CDC) said in releasing the data that “vaccination remains the safest strategy” against the disease.
She also noted that the disease exposes you to serious complications, while vaccines have been proven to be very safe and effective.
The study was conducted before steroid doses were widely available and before the emergence of the Omicron variant, which now accounts for more than 99 percent of new cases in the United States. So it is possible that the measures have shifted towards vaccination which is more effective than post-infection immunity.
However, the results published Wednesday provide important insights into the differences between vaccine-derived immunity and immunity after infection.
Health authorities studied cases in New York and California from late May to November 2021. The disease delta became the majority in the United States in late June.
During the entire study period, those who had not been vaccinated or had the disease in the past were by far the least protected.
But before Delta, vaccinated people who had never contracted COVID-19 were better protected than unvaccinated people who had already contracted the disease.
After delta was introduced, the ratio was reversed.
Less dangerous to infected but unvaccinated people
The study analyzed the risk of developing delta disease compared to the risk of those most likely to get it, in other words, people who had not been vaccinated in early October or who were infected in the past.
Those who had been vaccinated but did not contract the COVID-19 virus were six times less likely in California and nearly five times less likely in New York.
But the risk was lower for individuals who were previously infected but not vaccinated: at 29 in California and 15 in New York.
When analyzing hospitalization risks, this time only in California, the researchers found a similar reversal between the two time periods.
The CDC explained that “this may be due to different triggers of the immune response” that results from encountering the real virus or vaccine.
The study authors added that the decline also “coincided with the onset of vaccine-induced declines in immunity in many people” before booster doses.
The CDC also noted that work on Delta in other countries “also demonstrated increased protection for previously infected individuals, vaccinated or not, compared to vaccination alone.”
They emphasized that further studies are needed to examine the robustness of the protection offered by infection with both variants, including Omicron.
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