A study in northern Italy found infectious-disease antibodies persisted in detectable levels for at least nine months after infection, regardless of a symptomatic or asymptomatic course of illness, though results differed depending on test used.
Researchers from Imperial College London and the University of Padova published findings in Nature Communications on Monday, stemming from an analysis in Vo’, Italy, where a mass testing campaign saw 86% (2,602 people) of the community tested in February/March and May 2020, about 6% of whom tested positive and were tested again in November.
Results indicated 98.8% of COVID-positive individuals had detectable levels of antibodies by November, and nearly 20% had increased levels or reactivity since May, suggesting potential reinfection. Scientists tracked antibody levels through three tests manufactured by Roche, DiaSorin and Abbott and found differing rates of decay in antibody levels.
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“The May testing demonstrated that 3.5 percent of the Vo’ population had been exposed to the virus, even though not all of these subjects were aware of their exposure given the large fraction of asymptomatic infections,” Professor Enrico Lavezzo of the University of Padua said in a news release posted to EurekAlert.org on Monday.
“However, at the follow-up, which was performed roughly nine months after the outbreak, we found that antibodies were less abundant, so we need to continue to monitor antibody persistence for longer time spans.”
Other evidence has suggested antibodies linger at least six months after initial infection, while another team found detectable levels of antibodies 11 months later, claiming even a mild case of coronavirus could leave people with lifelong protection against the virus.
In the study at hand, lead author Dr. Ilaria Dorigatti, lecturer at Imperial College London, said the team “found no evidence that antibody levels between symptomatic and asymptomatic infections differ significantly, suggesting that the strength of the immune response does not depend on the symptoms and the severity of the infection.”
She urged caution over comparing infection estimates among populations administered different tests at various time intervals.
A further analysis of household contacts suggested a 1 in 4 probability that an infected individual would transmit the virus to others, and a minority of infections generated a large number of secondary infections.
“It is clear that the epidemic is not over, neither in Italy nor abroad,” Dorigatti said. “Moving forward, I think that it is of fundamental importance to continue administering first and second vaccine doses as well as to strengthen surveillance including contact tracing. Encouraging caution and limiting the risk of acquiring SARS-CoV-2 will continue to be essential.”
