• We don't know how long coronavirus antibodies last.
  • Recent research from Spain suggests they may disappear in some patients in a matter of weeks. Other studies suggest antibodies last a few months.
  • Here's what we do know so far about coronavirus antibodies' lifespans, and what it means for immunity and potential vaccines.
  • Visit Business Insider's homepage for more stories.

Among the many lingering questions about the coronavirus, one of the most crucial is: How long do antibodies last?

With some diseases, like measles or Hepatitis A, infection is a one-and-done deal. Once you get sick and recover, you're immune for life.

"For human coronaviruses, that's not the case," Florian Krammer, a vaccinologist at the Icahn School of Medicine at Mount Sinai, told Business Insider. "You can get repeatedly infected once your immunity goes down."

Increasingly, research is starting to coalesce around an unfortunate picture of COVID-19 immunity: People who develop antibodies might not keep them for very long.

Last month, a study revealed that antibodies may last only two to three months. Then research published Monday suggested that antibodies could only last three to five weeks in some patients.

Such findings have implications for vaccine development, since the efficacy of a vaccine hinges on the idea that a dose of weakened or dead virus can prompt your body to generate antibodies that protect you from future infection. If those antibodies are fleeting, a vaccine's protection would be fleeting, too. 

Short-lived antibodies also diminish hopes of achieving widespread or permanent herd immunity.

Antibodies may last just a few weeks or months

In April, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, hypothesized that people who were recovering from COVID-19 at that time would likely be immune into the fall. 

"If we get infected in February and March and recover, next September, October, that person who's infected — I believe — is going to be protected," Fauci said. 

But the evidence from the June study suggests the time frame might be shorter.

The researchers tested for antibodies in 37 people who had fallen ill and recovered in Wanzhou, China. They also tested 37 others who had tested positive for the virus but never showed symptoms. About eight weeks after recovery, antibodies dropped to undetectable levels in 40% of the asymptomatic people and in 13% of those who had symptoms.

The most recent research on this topic, however — published this week in The Lancet — suggests that one in five people studied lost detectable levels of antibodies within five weeks.

The study tested 60,000 people in Spain for antibodies three times between April and June. About 7% of the participants who had antibodies during the first phase of the study (April 27 to May 11) no longer had them in the second phase (May 18 to June 1), according to CNN. Roughly 14% of participants who had antibodies during the first stage no longer had them by the third phase (June 8 to June 22).

Loss of immunity was most common among people who never developed symptoms, Reuters reported.

"Immunity can be incomplete, it can be transitory, it can last for just a short time and then disappear," Raquel Yotti, one of the study co-authors, said during a news conference. 

But Krammer said he's skeptical of the findings. 

"It's physically not possible for antibodies to disappear in a matter of weeks," he said.

Krammer pointed out that the half life time for immunoglobin (IgG), the most common antibody found in our blood, is one to three weeks. That's how long it takes for the number of those protective proteins to decrease by half.

"Within weeks, the amount of antibodies you have will be cut in half, not gone," he said.

We don't know what level of antibodies protect us from infection

The problem with studying how long antibodies last is that it's still unclear what level one needs to be considered immune in the first place.

"We do not know how much protection the antibodies may provide or how long this protection may last," the Centers for Disease Control and Prevention warns.

With many better-studied viruses like measles, Krammer said, "you know how many antibodies you need to be protected."

But we don't have a specific number for the coronavirus. 

Identifying that threshold could be "extremely helpful for vaccine development," Krammer said — that way, researchers conducting trials would know whether their formula is going to work.

But even if antibodies disappear, that doesn't mean a vaccine is useless

The point of an effective vaccine is to produce antibodies in enough individuals to build up herd immunity within the population. To put the coronavirus in decline, at least 50% of the population would have to be immune, perhaps far more.

But it's not a deal-breaker if people become susceptible to reinfection sometime after the initial shot, Krammer said. 

"This happens for a lot of vaccines," he said. "It's not a problem, you can get revaccinated."

That's what booster shots are for. The question, however, is whether follow-up shots will need to happen on the scale of months or years.

Your immunity to the virus isn't solely tied to antibodies

There's one other reason that findings about antibody levels dropping shouldn't cause for excessive concern: Your immunity doesn't just depend on these proteins. 

White blood cells also have an impressive immunological memory that can help your body identify and attack the invading virus should it ever return. T cells can destroy infected cells, and B cells work to produce new antibodies.

"If you're reinfected after some time, it would be an attenuated disease. It will be not as severe as the first time because your B and T cells remember the virus and react quickly," Krammer said. 

Some research has even shown that not everyone who gets sick develops detectable antibodies at all, which suggests their response to the virus could be tied up in a different layer of the immune system.

But neither recent study of antibodies — in China or Spain — tested participants to see how many T cells and B cells they developed. 

Hilary Brueck, Morgan McFall-Johnsen, and Adam Payne contributed reporting to this story.

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