According to a new study six months after being infected from SARS-COV19, the blood of individuals who had mild, moderate or asymptomatic COVID-19 still contained immune cells that recognize the virus.

The study involves analysing multiple compartments of immune memory over time: antibodies, B cells and T cells, among other features of immune memory. The study included 185 adults, ages 19 to 81, in the United States who had recovered from COVID-19. Most of the adults had mild symptoms of COVID19.

The researchers – from the La Jolla Institute for Immunology, University of California, San Diego, and Icahn School of Medicine at Mount Sinai – analysed blood samples collected at various points following the onset of symptoms, with some collected more than six months later.

In the blood samples, the researchers examined components of immune memory. They found that antibodies “were durable” with only “modest declines” emerging at six to eight months, but noted that there was about a 200-fold range in the level of antibody responses among the adults.

The researchers also found that memory B cells were detected in almost all COVID-19 cases, and there appeared to be an increase in memory B cells over time. “B cell memory to some other infections has been observed to be long-lived, including 60+ years after smallpox vaccination, or 90+ years after infection with influenza,” the researchers wrote in their study.

The researchers identified two types of T cells and their data suggest that “T cell memory might reach a more stable plateau, or slower decay phase, later than the first 6 months post-infection,” they wrote.

Respiratory viruses such as influenza are trickier. People can catch flu over and over again and flu vaccines generally provide only partial protection against infection and severe disease. Part of that’s due to flu’s tendency to mutate.

Coronaviruses seem to fall in between. There are seven known Coronaviruses that can infect people and SARS-CoV-2 is just one of them. Many human Coronaviruses can cause the common cold, but because they are not usually deadly, they are not as well studied.