TOKYO (Kyodo) -- Antibodies in people infected by older versions of the novel coronavirus could become weaker in defending against COVID-19 variants over time, a recent study by a Japanese university showed.
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 percent of those with mild or no symptoms had antibodies against the coronavirus six months after being infected. One year later, 96 percent 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 percent 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 percent against the Indian strain, 81 percent against the Brazilian variant and 85 percent 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 percent 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.