Two studies found that antibodies can last for months after a Covid-19 infection, and suggested potential new methods for testing.
Two separate studies have found that antibodies that target the novel coronavirus (SARS-CoV2) have been found in Covid-19 patients at least three months after they first contracted the virus.
The first study, published to Science Immunology, took blood samples from 343 patients with Covid-19, most of whom had severe cases, up to four months after they first experienced symptoms. These samples were then compared with 1,548 control individuals sampled before the pandemic.
The researchers focused on antibodies specific to the SARS-CoV-2 spike protein’s receptor binding domain. They found that measuring an antibody called immunoglobulin G (IgG) was highly accurate in identifying infected patients who had symptoms for at least 14 days.
They also found that IgG levels remained elevated in patients for four months, and were associated with the presence of protective neutralising antibodies that also demonstrated little decrease in activity over time.
However, researchers found that people infected with SARS-CoV-2 had immunoglobulin A (IgA) and immunoglobulin M (IgM) responses that were relatively short-lived, declining to low levels within around two and a half months or less, on average.
“We can say now that if a patient has IgA and IgM responses, they were likely infected with the virus within the last two months,” said Richelle Charles of Massachusetts General Hospital, a senior author of the paper.
According to Charles, standard nasal swab tests for the also coronavirus lose sensitivity over time. However, by augmenting them with a test for antibodies in patients who have had symptoms for at least eight days, it could help identify some positive cases that might otherwise be missed.
Another study, also published to Science Immunology and led by the University of Toronto, found that while IgA and IgM antibodies rapidly decayed, IgG antibodies stayed relatively stable for up to 105 days after symptoms appeared. This was found by taking saliva samples and blood tests from 402 patients diagnosed with Covid-19.
Their antibody responses were charted from three to 115 days after symptom onset and compared with 339 pre-pandemic controls. Patients with Covid-19 showed peak IgG levels at 16 to 30 days after the appearance of symptoms.
“Our study shows that IgG antibodies against the spike protein of the virus are relatively durable in both blood and saliva,” said Jennifer Gommerman, professor of immunology at the University of Toronto and leader of the saliva testing effort.
“Our study suggests saliva may serve as an alternative for antibody testing. While saliva is not as sensitive as serum, it is easy to collect.”