Coronavirus antibodies last "at least" nine months after infection, according to Imperial College London and the University of Padua.
Antibody levels "remain high" whether or not the infected person developed symptoms of Covid-19, the disease caused by the virus, the researchers found, after testing patients in northern Italy, one of the hardest hit regions at the outset of the pandemic.
"The great majority of SARS-CoV-2 (coronavirus) infections, irrespective of symptom onset, develop antibodies," according to the research, which was published on Monday in the journal Nature Communications.
Around 3,000 residents of Vo, a northern Italian town sealed off by the Italian army in early 2020 after some of Europe's first local virus transmissions were found, were tested for antibodies in May 2020 and again in November.
By late last year "detectable levels of antibodies" remained in 98.8% of people who caught the virus in February or March.
The team said they even detected cases of antibody levels increasing, suggesting "potential re-infections" and a possible "boost to the immune system."
Most estimates, including by manufacturers, suggest coronavirus vaccines give at least six to eight months immunity against older variants, though Washington University research published last month by Nature suggested immunity could be "lasting," depending on the jab used.
"We need to continue to monitor antibody persistence for longer time spans," warned Enrico Lavezzo of the University of Padua, whose team's tests suggested a one-in-four probability of infections being spread within households and that around 80% of transmissions could be traced to just 20% of infections.