A new study of recovered coronavirus patients found that patients develop different levels of antibodies to the virus.

Ten patients of the 175 studied — 6% — didn't have any detectable coronavirus antibodies in their systems.

The study found that elderly and middle-aged people developed higher levels of antibodies.

Visit Business Insider's homepage for more stories.

A new study from Chinese scientists on 130 recovered COVID-19 patients is raising questions about the extent to which people develop immunity to the virus.

The paper — a pre-print that has not been peer-reviewed yet — found that patients produced differing levels of antibodies . Having identifiable coronavirus antibodies in your bloodstream means you've probably built up immunity. But roughly 6% of the patients studied didn't develop any detectable antibodies at all.

"What this will mean to herd immunity will require more data from other parts of the world," Huang Jinghe, the leader of the research team behind the report, said, according to the South China Morning Post.

Interestingly, the levels of antibodies patients produced seemed to correlate with their ages: Middle-aged and elderly recovered patients had higher levels of antibodies. Nine of the 10 of the patients who did not develop detectable levels of coronavirus antibodies were 40 years old or younger.

Finding out more about how antibodies to the virus work will have major implications for both vaccine development and the potential for herd immunity.

Measuring antibodies in recovered patients

A laboratory technician prepares COVID-19 patient samples for semi-automatic testing at Northwell Health Labs, Wednesday, March 11, 2020, in Lake Success, N.Y. The US Food and Drug Administration has approved faster testing protocols as the viral outbreak continues to spread worldwide. For most people, the new coronavirus causes only mild or moderate symptoms. For some it can cause more severe illness AP Photo/John Minchillo

The study, from researchers from Fudan University in Shanghai, took blood samples from 175 coronavirus patients who had recovered at Shanghai hospitals and who'd had "mild" symptoms. (Patients with "severe" symptoms were excluded because many had received blood transfusions to treat their illnesses.)

The participants ranged in age from 16 to 68, and the scientists grouped them into three categories: elderly (60-85), middle-aged (40-59), and young (15-39).

They found that the patients developed antibodies around 10 to 15 days after the disease's onset and remained stable afterwards.

The researchers measured the levels of neutralizing antibodies (NAbs) in each patient's blood, and found that recovered elderly patients developed significantly higher levels of antibodies than younger patients did. But there was no difference between the lengths of the patients' hospital stays.

"These results indicated that high level of NAbs might be useful to clear the viruses and helpful for the recovery of elderly and middle-age patients," the authors wrote.

The virus seems to be more fatal for older people. In the US, patients 65 and older are seeing the highest rates of death and serious illness, according to the Centers for Disease Control and Prevention.

Not everyone developed antibodies

In the 10 patients in the study who did not develop detectable antibodies, "other immune responses, including T cells or cytokines, may contribute to the recovery," the researchers wrote.

T cells are a type of white blood cells that aid in immune response, and cytokines are a type of molecule that cells release to fight infections. However, when too many cytokines are released, they cause inflammation — which has reportedly contributed to fatal outcomes in some COVID-19 patients.

Even in patients who do develop coronavirus antibodies, scientists still aren't sure how long they'll last; the virus has not been around long enough to study long-term effects.

Generally, once your body has antibodies to fight off a particular disease, you can't get it again, though some types of antibodies weaken over time. Plus, with viruses that mutate — such as the common cold or seasonal flu — antibodies people build up against one strain aren't effective against others

The possibility of reinfection and implications for vaccines

As the US looks to roll out antibody tests that could tell people whether they've already had the virus and developed immunity, the findings of the Shanghai study could be concerning.

But Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, has said it's unlikely that people would get the coronavirus more than once — at least within a short time period.

"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 on Wednesday during a livestreamed conversation with Howard Bauchner, editor of the Journal of the American Medical Association.

Over 375,000 people worldwide have recovered from the coronavirus (likely more, given that many mild and asymptomatic cases are not reported in official counts). Given that a third of the world is under some kind of lockdown, those who have recovered could potentially emerge and return to work first.

"Those are the people, when you put them back to particularly critical infrastructure jobs, that you worry less about them driving an outbreak than those who are antibody-negative and very likely have never been exposed," Fauci said.

Correction: A prior version of this story said the percentage of recovered patients who did not develop antibodies was 8%. It was actually 6%.