We don’t know enough about COVID antibodies to count on them

A new study raises more questions about how long immunity to COVID via antibodies lasts and whether you can get this disease multiple times.

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One of the biggest outstanding questions about the coronavirus, and one which could well determine the course of the pandemic, is the role that antibodies play in immunity.

By now you’ve probably heard plenty about antibodies in the context of immunity and developing a COVID vaccine. These Y-shaped proteins form because of an immune response to a pathogen or other hostile biological material. They may be personalized to sites on an individual virus called antigens, to which they attach and help prevent infection.

As with many viruses, antibodies form during the course of a COVID case and should offer protection against against a second coronavirus infection. But it’s still unclear just how potent these antibodies are and whether or not they may provide stronger immunity for some people more than others. And that raises the question of whether or not you can contract COVID-19 twice.

One central mystery that may take years to answer is how long COVID antibodies last. First off, antibodies don’t always behave the same everyone. Some may form powerful antibodies with staying power; others whose body’s so-called adaptive immune system produces weaker ones may face a much more brutal fight with COVID and be at a higher risk for reinfection. Human biology can react in unpredictable ways to new adversaries.

Then there’s the type of pathogen that the novel coronavirus is itself. Coronaviruses encompass a broad class of bugs which can include everything from some types of the common cold to SARS and MERS. There is to date no cure or vaccine for the common cold since there is such a variety of strains. There also aren’t any commercially available ones for SARS or MERS—or any coronaviruses for that matter.

Part of the reason for that is both the SARS and MERS outbreaks cause milder disease than COVID-19, and the former were contained relatively quickly. Patients who contracted SARS during that outbreak were found that have protective antibodies for an average of two years.

That doesn’t seem to be the case with the novel coronavirus—at least for certain COVID patients. In those who suffer from a mild case, antibody levels may be cut in half in just over two months, according to one study in the New England Journal of Medicine.

Another new analysis by Imperial College London scientists released this week examined antibody levels among the British population. The report found that antibody prevalence dropped sharply and quickly in the study population: from 6% in late June to 4.4% in late September.

“This very large study has shown that the proportion of people with detectable antibodies is falling over time,” said Helen Ward, one of the lead study authors.

But there’s another twist: Those falling antibody levels don’t necessarily mean you’ll be reinfected with COVID. “We don’t yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19, but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others,” she said.

Other studies have shown there is a non-zero number of people who have been reinfected after recovering from COVID. A Lancet report published two weeks ago examined the case of a 25-year-old man from Washoe County in Nevada who contracted COVID-19 once in April and again at the end of May.

“The degree of protective immunity conferred by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is currently unknown. As such, the possibility of reinfection with SARS-CoV-2 is not well understood,” wrote the authors from the University of Nevada and Nevada State Public Health Library.

To date, as the Centers for Disease Control (CDC) puts it, “confirmed and suspected cases of reinfection of the virus that causes COVID-19 have been reported, but remain rare​.”​

Experts stress that the mere presence of coronavirus antibodies is no reason to assume you’ll have long-lasting immunity or protect others from infection. The latter point throws a big wrench into proponents of a “herd immunity” approach wherein you simply let enough people get infected and become immune.

At the end of the day, it is still be important to wash your hands, wear a mask, socially distance, and be generally responsible—antibodies or otherwise.

More coronavirus coverage from Fortune:

• The COVID vaccine timeline: Where the candidates are now—and what’s still to come
• Pfizer’s high-tech plan to track every dose of its experimental COVID vaccine
• Don’t expect a quick COVID recovery for the global economy
• Fareed Zakaria: COVID-19 lockdowns are a sign of failure
• Commentary: We need a G.I. Bill for frontline workers, the heroes of COVID-19