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.
Nevertheless, “vaccination remains the safest strategy” once morest the disease, the Centers for Disease Control and Prevention (CDC) pointed out in releasing the data. Indeed, contracting the disease exposes you to serious complications, while vaccines have proven to be extremely safe and effective.
After Delta, the relationship reversed
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 following 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 the 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.
A risk divided by 29 in people infected but not vaccinated
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 regarding 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 might be due to the different stimulations of the immune response” caused by either encountering the real virus or a vaccine, the CDC explains.
This reversal also “coincided with the onset of vaccine-induced immunity waning in many people,” before the booster doses, the study authors add. The CDC notes that work on Delta in other countries “also demonstrated increased protection of previously infected, vaccinated, and unvaccinated individuals, compared to vaccination alone.” They stressed that further studies were needed to investigate the durability of protection conferred by infection once morest each of the variants, including Omicron.
