In a new study from the University of Padua and Imperial College London, researchers found antibody levels remain high nine months after SARS-CoV-2 infection, whether symptomatic or asymptomatic.
They tested more than 85% of the 3,000 residents of Vo’, Italy, in February/March 2020 for infection with SARS-CoV-2, the virus that causes COVID-19, and tested them again in May and November 2020 for antibodies against the virus.
The team found that 98.8% of people infected in February/March showed detectable levels of antibodies in November, and there was no difference between people who had suffered symptoms of COVID-19 and those that had been symptom-free.
Antibody levels were tracked using three ‘assays’ – tests that detect different types of antibodies that respond to different parts of the virus.
The results showed that while all antibody types showed some decline between May and November, the rate of decay was different depending on the assay.
The team also found cases of antibody levels increasing in some people, suggesting potential re-infections with the virus, providing a boost to the immune system.
The team says that the strength of the immune response does not depend on the symptoms and the severity of the infection.
But antibody levels vary, sometimes markedly, depending on the test used. This means that caution is needed when comparing estimates of infection levels in a population obtained in different parts of the world with different tests and at different times.
The team also investigated the infection status of household members, to estimate how likely an infected member is to pass on the infection within the household.
Their modeling suggests that there was a probability of about 1 in 4 that a person infected with SARS-CoV-2 passes the infection to a family member and that most transmission (79 percent) is caused by 20 percent of infections.
This finding confirms that there are large differences in the number of secondary cases generated by infected people, with the majority of infections generating no further infections and a minority of the infections generating a large number of infections.
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The study is published in Nature Communications. One author of the study is Dr. Ilaria Dorigatti.
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