The levels of IgG antibodies against SARS-CoV-2 remain stable, or even increase, seven months after infection, according to a study of healthcare workers published in the journal Nature Communications on Friday.
The researchers from the Barcelona Institute for Global Health (ISGlobal) in Spain also suggest that pre-existing antibodies against common cold coronaviruses could protect from COVID-19.
They noted that in order to predict the pandemic's evolution and develop effective strategies, it is critical to better understand the dynamics and duration of immunity to SARS-CoV-2.
The researchers followed a group of healthcare workers from the beginning of the pandemic in order to evaluate the levels of antibodies against different SARS-CoV-2 antigens over time.
"This is the first study that evaluates antibodies to such a large panel of SARS-CoV-2 antibodies over 7 months," said ISGlobal researcher Carlota Dobano, who led the study.
The team analysed blood samples from 578 participants, taken at four different timepoints between March and October 2020.
They measured the level and type of IgA, IgM or IgG antibodies to six different SARS-CoV-2 antigens as well as the presence of antibodies against the four coronaviruses that cause common colds in humans.
The researchers also analysed the neutralising activity of antibodies in collaboration with researchers at the University of Barcelona.
The results show that the majority of infections among healthcare workers occurred during the first pandemic wave.
With the exception of IgM and IgG antibodies against the nucleocapsid (N), the rest of IgG antibodies remained stable over time, confirming results from other recent studies.
Immunoglobulin G (IgG), the most abundant type of antibody, is found in all body fluids. These antibodies are produced in the later stages of an infection and see the body through recovery.
"Rather surprisingly, we even saw an increase of IgG anti-spike antibodies in 75 per cent of the participants from month five onwards, without any evidence of re-exposure to the virus," said Gemma Moncunill, senior co-author of the study.
No reinfections were observed in the cohort, the researchers said.
The study also suggests that the antibodies against human cold coronaviruses (HCoV) could confer cross-protection against COVID-19 infection.
People who were infected by SARS-CoV-2 had lower levels of HCoV antibodies, according to the researchers. Also, asymptomatic individuals had higher levels of anti-HCoV IgG and IgA than those with symptomatic infections, they said.
"Although cross-protection by pre-existing immunity to common cold coronaviruses remains to be confirmed, this could help explain the big differences in susceptibility to the disease within the population," Carlota Dobano added.
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