COVID-19 antibodies persist for at least nine months after infection

Tests from an entire Italian city show that antibody levels remain high nine months after SARS-CoV-2 infection, whether symptomatic or asymptomatic.

Researchers at the University of Padua and Imperial College London tested more than 85 per cent of the 3,000 residents in Vo ‘, Italy, in February / March 2020 to detect infection with SARS-CoV-2, the virus that causes the 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 were no differences between people who had symptoms of COVID-19 and those who had no symptoms. . The results are published today in Communications on Nature.

Antibody levels were monitored using three assays: tests that detect different types of antibodies that respond to different parts of the virus. The results showed that, although all types of antibodies showed some decrease 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 possible re-infections with the virus, providing a boost to the immune system.

The lead author, Dr. Ilaria Dorigatti, of the MRC Center for Global Infectious Disease Analysis and the Abdul Latif Jameel Institute for Disease and Emergency Analytics (J-IDEA) of the Imperial, said: they differ significantly, suggesting that the strength of the immune response does not depend on the symptoms and severity of the infection.

“However, our study shows that antibody levels vary, sometimes remarkably, depending on the test used. This means that caution should be exercised when comparing estimates of infection levels in a population obtained in different parts of the world. with different tests and at different times. “

Professor Enrico Lavezzo, of the University of Padua, said: “Tests in May showed that 3.5 per cent of the population of Vo ‘had been exposed to the virus, although not all of these subjects were aware of its exposure given the large fraction of asymptomatic.infections.

“However, in the follow – up, which took place about nine months after the outbreak, we found it antibodies they were less abundant, so we need to continue to monitor the persistence of antibodies for longer periods of time. “

The team also investigated the status of infection in household members to estimate the likelihood that an infected member would transmit the infection to the home. Its modeling suggests that there was a probability of approximately 1 in 4 people being infected with SARS-CoV-2. infection to a family member and that most of the transmission (79%) is caused by 20% of infections.

This finding confirms that there are large differences in the number of secondary cases generated by infected people, most infections do not generate more infections, and a minority of infections generate a large number of infections.

The large differences in how an infected person can infect other people in the population suggest that behavioral factors are key to controlling the epidemic, and that physical distancing, in addition to limiting the number of contacts and use of masks, remains important to reduce the risk of transmitting the disease, even in highly vaccinated populations.

The team’s data set, which includes the results of two massive PCR testing campaigns conducted in February and March and the antibody survey conducted in May and again in November, also allowed them to be eliminated. the impact of various control measures.

They showed that in the absence of case isolation and short closures, manual tracking of contacts would not have been sufficient to suppress the epidemic.

The project’s lead professor, Andrea Crisanti, of the Imperial Department of Life Sciences and the Department of Molecular Medicine at the University of Padua, said: “Our study also shows that manual contact tracking: search of positive individuals according to the information known and declared contacts — it would have had a limited impact on the containment of the epidemic, if it had not been accompanied by a massive screening ”.

Dr. Dorigatti added: “It is clear that the epidemic is not over, neither in Italy nor abroad. To move forward, I think it is vital to continue administering the first and second doses of vaccine, as well as to strengthen the surveillance, including contact tracking. Encouraging caution and limiting the risk of acquiring SARS-CoV-2 will continue to be essential. ”