The study, published in the journal Science, points to the IgG class of antibodies as the longest-lasting antibodies detectable in the patients during this time frame, and may serve as promising targets to detect and evaluate immune responses against the SARS-CoV-2 virus.

 

"These antibodies could be detected at similar levels in both blood and saliva suggests that saliva could be used as an alternative biofluid for antibody testing," said study authors, including those from Harvard University in the US.

 

In the first study, Anita Iyer and her team measured antibody responses in the blood of 343 patients with Covid-19 for up to 122 days after symptom onset -- and compared these responses to those of 1,548 control individuals sampled before the pandemic.

 

The researchers focused only on antibodies specific to the SARS-CoV-2 spike protein's receptor-binding domain.

 

To provide a baseline, the researchers estimated sensitivities of IgG, IgA, and IgM antibody types at 95 per cent, 90 per cent and 81 per cent, respectively, for detecting infected individuals between 15 to 28 weeks after symptom onset.

 

Among these antibodies, spike protein-specific IgM and IgA were short-lived, dropping beneath detection levels at around 49 and 71 days, respectively, after the appearance of symptoms.

 

In contrast, spike protein-targeted IgG responses decayed slowly over a period of 90 days, with only three individuals losing them within this timeframe.

 

Levels of spike protein-specific IgG strongly correlated with levels of neutralizing antibodies in the patients.

 

The researchers also did not observe cross-reactivity of any SARS-CoV-2-targeting antibodies with other "common cold" coronaviruses.

 

Similar to Iyer's research, another study found that while IgA and IgM antibodies targeting the spike protein's receptor-binding domain rapidly decayed, IgG antibodies remained relatively stable for up to 105 days after symptom onset in 402 patients with Covid-19.

 

The researchers detected spike protein-specific antibodies in the saliva, as well as the blood, of these patients.