• Antibody tests can measure whether a person's body has developed a protective response to an illness, like the coronavirus.
  • Around the world, antibody studies suggest that in most places, fewer than 1 in 10 people may have some degree of protection from another coronavirus infection.
  • "The problem with antibody testing right now is that we don't know what it means," one vaccine scientist and virus expert told Business Insider. 
  • Visit Business Insider's homepage for more stories.

Millions of people around the world have gotten sick with the novel coronavirus, but even those who've successfully recovered from an illness may not be immune to new infections.

COVID-19 spread globally

Coronavirus antibody tests — not to be confused with the nose and throat saliva tests that register an ongoing illness in the body — are blood draws that aim to measure whether people have been previously infected with the coronavirus.

These antibody tests determine whether an individual has mounted a protective immune response to the coronavirus, by producing y-shaped proteins called immunoglobulins. 

The tests still aren't completely accurate, but they're being dispatched in countries around the world as one initial marker of how many people may have gotten sick so far. 

The results of these antibody tests, while still preliminary, suggest that the world has quite a long way to go before its anywhere near immune to this virus, even if an illness may confer some degree of protection from another coronavirus infection (something scientists aren't sure about just yet). 

Here's a sampling of recent antibody test results from cities, counties, and countries around the world. You can see how hard-hit northern Italy and New York City have much larger percentages of positive antibody tests than most other spots around the globe, where still only a fraction of the population — fewer than one in ten people in most places — has mounted a detectable immune response to this virus:

"I think the problem with antibody testing right now is that we don't know what it means," Florian Krammer, a vaccine scientist and virus expert at the Icahn School of Medicine at Mount Sinai in New York, told Business Insider.

"We know that from other coronaviruses, from regular human common cold coronaviruses, you probably get immunity for some time, once you're infected, once you make antibodies, it's probably not so easy to get reinfected … but it's not perfect." 

One recent antibody study in Spain suggested that some people's immune responses to the virus are already waning, just months after infection. Another preprint (not peer-reviewed) study of coronavirus infections in China suggested some patients (especially those younger than 40) may not develop antibodies at all. 

"The critical question is how much antibodies do you need, in order to be protected?" Krammer said. 

More and more evidence is pointing to the idea that herd immunity, whereby enough people have developed protective antibodies to an illness like the coronavirus, either through infection or vaccination, is not something that's going to be possible without some shots. 

"There's not much we can do until there's a vaccine, and with a vaccine you're able to reach herd immunity very quickly," Krammer said. 

America's top infectious disease expert seems to agree with him on that. 

"I think the durable solution to what we're in right now clearly has to be a vaccine," Dr. Anthony Fauci told The Hill on Thursday. 

"We hope that by the end of this calendar year and the beginning of 2021, that we will have a vaccine that we'll be able to begin to deploy." 

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