What Immunity to COVID-19 Really Means ?
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The U.S. Food and Drug Administration recently granted an “emergency use authorization” of a blood test for antibodies against SARS-CoV-2, the novel coronavirus that causes COVID-19. It comes at a time when health experts and leaders are embracing immunity as a potential endpoint to the pandemic.

With some pathogens, such as the varicella-zoster virus (which causes chickenpox), infection confers near-universal, long-lasting resistance. Natural infection with Clostridium tetani, the bacterium that causes tetanus, on the other hand, offers no protection—and even people getting vaccinated for it require regular booster shots. On the extreme end of this spectrum, individuals infected with HIV often have large amounts of antibodies that do nothing to prevent or clear the disease.

At this early stage of understanding the new coronavirus, it is unclear where COVID-19 falls on the immunity spectrum. Although most people with SARS-CoV-2 seem to produce antibodies, “we simply don’t know yet what it takes to be effectively protected from this infection,” says Dawn Bowdish, a professor of pathology and molecular medicine and Canada Research Chair in Aging and Immunity at McMaster University in Ontario. Researchers are scrambling to answer two questions: How long do SARS-CoV-2 antibodies stick around? And do they protect against reinfection?

Immunity to seasonal coronaviruses (such as those that cause common colds), for example, starts declining a couple of weeks after infection. And within a year, some people are vulnerable to reinfection. That observation is disconcerting when experts say it is unlikely we will have a vaccine for COVID-19 within 18 months. But studies of SARS-CoV—the virus that causes severe acute respiratory syndrome, or SARS, which shares a considerable amount of its genetic material with SARS-CoV-2—are more promising. Antibody testing shows SARS-CoV immunity peaks at around four months and offers protection for roughly two to three years.

Even if the antibodies stick around in the body, however, it is not yet certain that they will prevent future infection. What we want, Bowdish says, are neutralizing antibodies. These are the proteins that reduce and prevent infection by binding to the part of a virus that connects to and “unlocks” host cells. They are relatively easy to detect, and they are far easier for vaccine developers to generate than the alternative: the immune system’s T cells. In contrast, nonneutralizing antibodies still recognize parts of the pathogen, but they do not bind effectively and so do not prevent it from invading cells.

“If humans naturally make neutralizing antibodies [against SARS-CoV-2], then all we have to do is figure out what [sites they are] binding on the virus and really target that one little piece of protein, and that’s our magic bullet,” Bowdish says. For SARS-CoV-2, that target site is most likely on the so-called receptor-binding domain of its spike glycoprotein—a protein attached to a sugar that the virus uses to enter cells. But, Bowdish says, this spot may present a challenge because human immune systems are not very good at making antibodies against sugar-coated substances.

Research on real-life immunity to SARS-CoV-2 is in its preliminary stages, and uncertainties remain. One study found no correlation between viral load and antibody presence, leading the authors to question the antibodies’ actual role in clearing the virus in humans. In addition, peer-reviewed research on SARS-CoV and preprint studies on SARS-CoV-2 report that some nonneutralizing coronavirus antibodies might trigger a harmful immune response upon reinfection with those pathogens or cross-infection with other coronaviruses. Thus, while much of the emerging research is promising,

In an ideal world, SARS-CoV-2 immunity would resemble that acquired by children who get chickenpox. Early research suggests we are in for a much more complex scenario but one that time and unprecedented global cooperation might be able to untangle. Eventually, antibody tests could be the key to getting our lives and economies back on track. For now, they promise to give experts, officials, and citizens a clearer picture of the pandemic.

Source: https://www.scientificamerican.com/article/what-immunity-to-covid-19-really-means/
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