WASHINGTON | Against the Delta variant, unvaccinated people who had previously contracted COVID-19 were better protected than people who were only vaccinated, according to a study by American health authorities published on Wednesday.
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Despite everything, “vaccination remains the safest strategy” against the disease, stressed the Centers for Disease Prevention and Control (CDC) in publishing these data. Indeed, contracting the disease exposes you to serious complications, while vaccines have proven to be extremely safe and effective.
This study was also conducted before booster doses became widely available, and before the appearance of the Omicron variant, which now accounts for more than 99% of new cases in the United States. It is therefore possible that the cards are currently being reshuffled.
However, these results provide key elements for better analyzing the differences between immunity acquired by vaccines or after an infection.
Health authorities studied the cases in the states of New York and California, from the end of May to November 2021. Delta became majority in the United States at the end of June.
Throughout the period analysed, the least well-protected people were by far those who had neither been vaccinated nor had fallen ill 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 had already fallen ill.
After Delta arrived, the relationship reversed.
The study analyzed, for the beginning of October, the risk of catching Delta compared to that incurred by those most likely to contract it, namely people who have not been vaccinated or infected in the past.
Vaccinated individuals (but never infected) thus had six times less risk than they of catching it in California, and about five times less in New York. But this risk was even further reduced for people previously infected (but not vaccinated): by 29 in California, and by 15 in New York.
Analyzing the risk of hospitalization, this time in California only, the researchers found a similar reversal between the two periods.
How to explain it? “This could be due to the different stimulations of the immune response” caused by either the encounter with the real virus or a vaccine, explain the CDC.
This reversal also “coincided with the onset of vaccine-induced immunity decline in many people,” before the booster doses, the study authors add.
The CDC notes that work on Delta in other countries “has also demonstrated increased protection of previously infected, vaccinated, and unvaccinated people, compared to vaccination alone.”
They stressed that further studies were needed to investigate the durability of protection conferred by infection against each of the variants, including Omicron.