Long-lived antibodies in both blood and saliva of patients with COVID-19 detected

Two separate studies have documented the persistence of antibodies that target SARS-CoV-2 in hundreds of patients with COVID-19 at least 3 months after symptom onset. Both studies point to the IgG class of antibodies as the longest-lasting antibodies detectable in the blood and saliva of patients during this timeframe, suggesting that SARS-CoV-2-specific IgG antibodies may serve as promising targets to detect and evaluate immune responses against the virus. That 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.

In the first study, the authors 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%, 90%, and 81%, 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 3 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.

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. They charted the patients' antibody responses from 3 to 115 days after symptom onset, and compared their profiles with 339 pre-pandemic controls.

Patients with COVID-19 showed peak IgG levels at 16 to 30 days after the appearance of symptoms. Levels of all spike protein-specific IgG, IgM, and IgA antibodies in the blood positively correlated with levels observed in matched saliva samples. "Given that the virus can also be measured in saliva by PCR, using saliva as a biofluid for both virus and antibody measurements may have some diagnostic value," the authors say.

https://immunology.sciencemag.org/content/5/52/eabe0367

https://immunology.sciencemag.org/content/5/52/eabe5511

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