Despite everything, “vaccination remains the safest strategy” against the disease, as the Centers for Disease Prevention and Control (CDC) emphasized by publishing this data. Indeed, contracting the disease exposes you to serious complications, while vaccines have proven to be very safe and effective.
This study was also conducted before stimulant doses became widely available, and before the emergence of the Omicron variant, which now accounts for more than 99% of new cases in the United States. So it is possible that the cards are currently modified.
Nevertheless, these results provide the essential elements for a better analysis of the differences between immunity acquired by vaccines or after infection.
Health authorities investigated cases in New York and California from late May through November 2021. Delta became the majority in the United States in late June. Throughout the analyzed period, the people least well protected were those who had not been vaccinated and had not had the disease in the past.
But before Delta, people who had been vaccinated and had never contracted Covid-19 were better protected than people who had not been vaccinated but were already infected. After the arrival of Delta, the relationship was reversed.
The study, at the beginning of October, analyzed the risks of catching deltas compared to those of those most at risk, that is, people who had not been vaccinated or infected in the past.
Thus, individuals who have been vaccinated (but never infected) have a six times lower risk of contracting it in California, and about five times lower in New York. But that risk was reduced even more for people who were previously infected (but not vaccinated): by 29 in California, and by 15 in New York.
How do we explain it?
By analyzing the risks of hospitalization, this time only in California, the researchers found a similar reversal between the two periods.
How do we explain it? “This may be due to different stimulation of the immune response” produced by either encountering the real virus or the vaccine, the CDC explains.
The study authors add that this reversal also “coincided with the onset of vaccine-induced decline in immunity in many people” prior to booster doses.
study to verify
The CDC notes that work on Delta in other countries “also showed increased protection for previously infected, vaccinated and unvaccinated people, compared to vaccination alone.”
They stressed that further studies are needed to verify the robustness of the protection conferred by the infection against each of the variants, including Omicron.