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French study finds contracting Covid gives 6-month immunity

The study looked at people who had contracted the virus naturally and found they have high levels of immunity for at least six months

Research published by a laboratory in Toulouse has found that people who contract the Covid-19 virus naturally, for the first time, have an 85% higher immune level against the virus for at least six months afterwards, compared to people who have not had the virus.

The study, which tested 8,758 blood samples from health workers between June 10-July 10, 2020, has been heralded as hopeful news for the health pandemic.

The results were published in the journal Clinical Infectious Diseases.  

Researchers study Covid exposure 

Dr Chloé Dimeglio, biostatistician in the laboratory of virology at the Toulouse CHU where the study was conducted, told news source La Dépêche that researchers wanted “get a snapshot” of the population after the first confinement in France to compare with data collected later when people were “supposed to have been exposed to higher levels of the virus”.

After the first confinement, they found only 3% of healthcare workers researchers tested had contracted the SARS-CoV-2 virus that causes Covid-19.

Experience of the 276 who tested positive (from a total pool of 8,758) varied – some had symptoms while others did not and others knew they had been exposed to the virus, while some had no idea.

Researchers then tested whether this “positive” group had developed antibodies against the virus and found that they had. Dr Dimeglio said: “At least 95% of the group were carrying antibodies but in different concentrations from one person to another.

"The logical next step was to track them over time to see how long their immunity to the virus lasted.”


Second tests reveal high immunity 

The group was tested for a second time in November 2020, five to six months after participants had given their first blood samples.

Dr Dimeglio said: “For 96.7% of the group, their level of antibodies had stayed stable or risen. This means that immunity acquired after a first infection has lasted for at least six months – which is rather good news.”

Reinfection rates also studied

The researchers also tracked workers who initially tested negative for Covid-19, but later took tests either because they had symptoms or believed they were a contact case.

This allowed them to compile data on initial infection rates, which they could compare with reinfection rates among those who had already tested positive.  

They found first-time infection rates to be at 12.1% among the group, whereas reinfection rates were just 1.8% (representing five cases in total).

But Dr Dimeglio said the study could not establish any rules for the amount of antibodies needed to prevent reinfection happening. “Equally, there does not seem to be a link between whether or not one has had symptoms or not”, she added. 

Vaccine still more effective

While results confirming natural immunity have been welcomed as good news, Dr Dimeglio still advises the Covid vaccine as the best protection against the virus.  

While the study found that antibodies gave 85% immunity for six months, this is inferior to the 95% immunity offered by vaccines such as those made by Pfizer and Moderna. 

“So, it is in our interest to get vaccinated,” Dr Dimeglio said. 

The study is currently scheduled to continue for a further two months, to track Covid immunity over a longer period.

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