TOKYO -- A year after their recovery from coronavirus infections, some 97% of people in a study still had antibodies that neutralize the conventional strain, according to results announced by a Yokohama City University research team on May 20.
But, the team also found that the antibody prevalence rate was lower among individuals with mild or no symptoms, and that the proportion of those with antibodies effective against mutant strains was also lower than those with antibodies that could fight the conventional strain.
Takeharu Yamanaka, a professor of biostatistics at the university, said, "Even if there are enough antibodies to prevent infection after a year, the overall amount is decreasing. It is presumably the same for vaccinations, so it may be desirable to revaccinate after about a year."
During viral infection or following vaccination, antibodies that attach to the invading virus are produced in the body to prevent human cells from being infected. It's not known in many cases how long antibodies last, and it appears related to vaccination frequency.
To find out more, the research team got cooperation in March and April from 250 people aged 21 to 78 who had been infected with and recovered from the coronavirus, and collected and analyzed their blood samples one year after infection.
All the test subjects were confirmed to have contracted the conventional strain between February and April 2020; 72.8% had no or mild symptoms, 19.6% were moderate cases, and 7.6% endured severe symptoms.
Their analysis showed that 96% of people with no or mild symptoms and 100% of former moderate and severe symptoms patients still had antibodies to neutralize the conventional strain a year on from infection -- making for a 97% overall figure. At six months after infection, 98% of overall individuals tested had such antibodies.
Researchers also looked for mutant strain antibodies; 90 to 98% of people with moderate or severe symptoms had them, while the figures were generally lower for individuals with no or mild symptoms: 79% for the British strain, 76% for the Brazilian strain, 69% for the South African strain, and 69% for the Indian strain.
The team explained that the lower antibody prevalence rates among people with no or mild symptoms may be due to fewer antibodies produced at infection, which could increase reinfection risks.
Generally, the presence of antibodies reduces chances of infection. In its analysis, the team used independently developed virus-like particles that resemble the coronavirus's shape, and did experiments using cultured cells for the body. The team says it cannot evaluate how much infection can actually be prevented.
(Japanese original by Ryo Watanabe, Science & Environment News Department)