More than a year and a half since the SARS-CoV-2 virus was first identified, researchers are still learning more about how the immune system responds to the virus. The question of how long antibodies remain in the body after recovery from a COVID-19 infection has been of interest for understanding reinfection risks and other aspects of the pandemic. Researchers from the University of Padua and Imperial College London have found that antibodies can persist for at least 9 months after infection, after screening more than 2,500 residents of an Italian town using a variety of assays and test methods.
The researchers first conducted PCR screening of the town of Vo’, Italy, in February and March of 2020, testing more than 85% of the town’s 3,000 residents. The researchers then tested the residents again in May 2020 using three different immunological assays to detect antibodies against the S and N antigens. In November 2020, those who had tested positive for COVID-19 in February, March or May were retested using the three serological assays to see whether antibodies could still be detected.
Of those who had tested positive in February or March, 98.8% showed detectable antibody levels in November, with no difference between those who had been symptomatic or asymptomatic during their infection. The researchers found that antibody levels tended to decrease between May and November, although some participants’ antibody levels increased, suggesting possible reinfection.
The screenings also showed that the three assays performed differently, with the calculated rate of decay of the antibodies differing between assays. A chemiluminescence immunoassay (CLIA) test recognizing the S antigen showed lower sensitivity and positive predictive value than the two other assays that recognized the N antigen and used electro-chemiluminescence immunoassay (ECLIA) and chemiluminescent microparticle immunoassay (CMIA) methods. However, the CMIA test showed a more marked decline in antibody titres and seropositivity between May and November compared to the other two tests. The research was published in Nature Communications.
“We 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,” said lead author Ilaria Dorigatti. “However, our study does show that 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 researchers also used surveys to track infections and exposures within households and found there was about a 25% chance that an infected person would pass the infection to a family member. They concluded from their data that about 79% of transmissions resulted from a minority (20%) of initial infections, and recommended that contract tracing be combined with case isolation, short lockdowns and mass screening.
Photo: A researcher testing a child as part of the study. Credit: Andrea Crisanti