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 US health authorities published this Wednesday.
Despite everything, “vaccination remains the safest strategy” against the disease, underlines the Centers for Disease Prevention and Control (CDC) when publishing these data. In fact, contracting the disease exposes you to serious complications, while vaccines have been shown to be extremely safe and effective.
This study was also done before booster doses were widely available and before the appearance 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 being shuffled right now.
However, these results provide key elements to better analyze the differences between immunity acquired by vaccines or after an infection.
Health authorities studied cases in the states of New York and California from the end of May to November 2021. Delta became a majority in the United States in late June.
Throughout the period analysed, the least protected people are, by far, those who have not been vaccinated or have become ill in the past.
But before Delta, people who had been vaccinated and had never gotten Covid-19 were better protected than people who hadn’t been vaccinated but had already gotten sick.
After Delta’s arrival, the relationship was reversed.
The study looked at, as of early October, the risk of contracting Delta compared to those most likely to contract it, that is, people who have not been vaccinated or infected in the past.
People who were vaccinated (but never infected) had six times less risk of contracting it than in California, and about five times less in New York. But this risk was further reduced for people previously infected (but not vaccinated): 29 in California and 15 in New York.
Looking at hospitalization risk, this time just in California, the researchers found a similar reversal between the two periods.
Variants of Covid-19 in the United States (AFP/Files – Eléonore HUGHES)
How to explain? “This could be due to the different stimulations of the immune response” caused by either the encounter with the real virus or a vaccine, the CDC explains.
This reversal also “coincided with the onset of vaccine-induced decline in immunity in many people” before booster doses, the study authors add.
The CDC notes that work on Delta in other countries “has also shown greater protection for 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.
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