Covid antibodies remain strong up to nine months after infection, regardless of whether a person had symptoms, a new study has found.
Almost an entire Italian town was tested by researchers from the University of Padua and Imperial College London to check how long antibodies last.
More than 85 per cent of the 3,000 residents of Vo', Italy, in February and March 2020 were tested for Ovid-19 infection, then tested again in May and November 2020 for antibodies against the virus.
The team found that 98.8 per cent of infected people showed detectable levels of antibodies up to nine months later. There was no difference between people who had suffered Covid symptoms and those who were symptom-free. The results were published on Monday in Nature Communications.
Previous studies suggested immunity lasts around six months, meaning it is possible to catch Covid twice. Being vaccinated reduces the likelihood of infection and douses symptoms but does not necessarily prevent falling ill.
The researchers previously found that mass testing and quarantining in Vo’ – which experienced the first known Covid-19 death in Italy – enabled the community to suppress the virus in only a few weeks. The town went into quarantine immediately after the death on February 21, 2020 and locking down the area and implementing a test and trace system “substantially reduced transmission”.
In the latest study the team found cases of antibody levels increasing in some people, suggesting potential reinfection with the virus, providing a boost to the immune system.
"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," lead author Dr Ilaria Dorigatti, of Imperial College said.
"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."
Prof Enrico Lavezzo, from the University of Padua, said: "The May testing demonstrated that 3.5 per cent 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.
"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."
The team found that there was a 25 per cent probability that a person would pass the infection to a family member and that most transmission (79 per cent) was caused by 20 per cent of infections.
The large differences in how one infected person may infect others suggests that behavioural factors are vital for epidemic control and physical distancing, as well as limiting the number of social contacts and mask-wearing, continue to be important to reduce the risk of transmitting the disease, even in highly vaccinated populations.