The findings by a Yokohama City University research team suggest that people who were infected in the early days of the pandemic, particularly those who had mild or no symptoms, still need to be vaccinated to reduce the risk of being infected by variants from Britain, South Africa, Brazil and India.

In the study, conducted on 250 people between the ages of 21 and 78, who tested positive for Covid-19 between February and April last year, 97% of those with mild or no symptoms had antibodies against the coronavirus six months after being infected. One year later, 96% of them still had antibodies to the disease.

Those who showed more severe symptoms all had Covid-19 antibodies after one year.

As for the variants, 69% of those who experienced mild or no symptoms had antibodies to fight off infections to the mutated strain from South Africa six months later, 75% against the Indian strain, 81% against the Brazilian variant and 85% against the British type, the study showed.

The percentages declined slightly more than one year later.

Antibody levels in the blood of former patients who had moderate to severe symptoms weakened only slightly against the virus variants in one year, according to the study.

Through 12 months after testing positive, at least 90% of such people had antibodies effective enough to block any of the four types of variants from infecting cells.

The study suggests that people with mild or no Covid-19 symptoms do not trigger an antibody response as strong those who develop a more serious illness, the team said.