Previous exposure to viruses has an impact on immunity. Broad serological profiling studies estimate that at any given time, a person carries antibodies against 10 different viral species. SARS-CoV-2 belongs to the betacoronavirus genus of coronaviruses, which includes SARS-CoV-1, MERS-CoV, and two other human coronaviruses (HCoV-OC43 and HCoV-HKU1), which are responsible for some forms of the “common cold.” The betacoronaviruses can induce immune responses against one another.

Virus neutralizing antibodies bind to a virus and prevent it from infecting a cell usually by preventing it from binding to a receptor. Previous studies have found that SARS-CoV-1 can generate neutralizing antibodies against HCoV-OC43 which would offer cross-immunity and HCoV-OC43 can generate cross-reactive antibodies against SARS-CoV-1. Additionally, antibodies from sera from people with the HCoV-OC43 coronavirus responsible for the common cold were found to cross-react with SARS-CoV-2 antibodies. It was also found that people infected with SARS-CoV-2 also increased their pre-existing antibodies against the classic cold coronavirus.

A couple of studies have investigated the effects of convalescent sera from patients that have recovered from the original SARS and whether the antibodies present in the sera can neutralize the SARS-CoV-2 virus. One study showed that convalescent sera from SARS patients that were harvested 3.5 years post-infection did cross-neutralize SARS-CoV-2 entry into a cell via the ACE2 receptor. However, another study found that sera from patients that had recovered from SARS-CoV-1 that was harvested only 22 days after infection could not neutralize the SARS-CoV-2 virus. It is unclear why there are conflicting results but there is one key difference between these studies. A major difference between the two studies is when the convalescent sera were collected from patients. In the study where there was an indication of cross-immunity, samples were collected 3.5 years after infection. However, in the study where there was no cross-immunity samples were collected within 22 days post-symptom onset, which is fairly early in the maturation of the humoral immune response when antibody titers are still increasing.

Some other research demonstrated that SARS-CoV-2-reactive CD4+ T cells were present in 40 to 60 percent of people that were never exposed to the SARS-CoV-2 virus, suggesting cross-reactive T cell recognition between circulating common cold coronaviruses and SARS-CoV-2. The results suggest that there could be some lingering immunity from the common cold but more research is needed to confirm.