ANTIBODY FIND BUOYS LAB QUEST TO FIND VIRUS CURE
Hopes of building ‘shield immunity’ get boost from new study.
Anew study offers a glimmer of hope in the grim fight against the coronavirus: Nearly everyone who has had the disease — regardless of age, sex or severity of illness — makes antibodies to the virus. The study, posted online on Tuesday, also hints that anyone who has recovered from infection may safely return to work — although it is unclear how long their protection might last. “This is very good news,” said Angela Rasmussen, a virologist at Columbia University in New York who was not involved with the work.
Antibodies are immune molecules produced by the body to fight pathogens. The presence of antibodies in the blood typically confers at least some protection against the invader.
Health officials in several countries, including the United States, have hung their hopes on tests that identify coronavirus antibodies to decide who is immune and can go back to work. People who are immune could replace vulnerable individuals, especially in high-transmission settings like hospitals, building what researchers call “shield immunity” in the population.
But most antibody tests are fraught with false positives — picking up antibody signals where there are none. The new study relied on a test developed by Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai, that has a less than 1% chance of producing false-positive results.
Several small studies have given reason to hope that people who have had Covid-19, the illness caused by the coronavirus, would gain some immunity for some period of time. The new study is the largest by far, with results from 1,343 people in and around New York City.
The study also eased a niggling worry that only some people — only those who were severely ill, for example — might make antibodies. In fact, the level of antibodies did not differ by age or sex, and even people who had only mild symptoms produced a healthy amount.
Having antibodies is not the same as having immunity to the virus. But in previous research, Dr Krammer’s team has shown that antibody levels are closely linked with the ability to disarm the virus, the key to immunity.
Researchers at Mount Sinai tested people who signed up to be donors of convalescent plasma, antibodies extracted from blood. The project has enrolled more than 15,000 people so far, according to Dr Ania Wajnberg, who is leading the effort.
The team tested 624 people who had tested positive for the virus and had recovered. At first, just 511 of them had high antibody levels; 42 had low levels; and 71 had none. When 64 of the subjects with weak or no levels were retested more than a week later, however, all but three had at least some antibodies.
That suggests the timing of testing for antibodies can greatly affect the results, the researchers said. “We weren’t looking exactly at this, but we had enough to say that 14 days is probably a little too early,” Dr Wajnberg said.
There was even a difference between levels at 20 days versus 24 days, she said, suggesting the optimal time for an antibody test is well after symptoms begin. “What we’re telling people now is at least three weeks after symptom onset,” Dr Wajnberg said.
Because tests to diagnose coronavirus infection were unavailable to most people in New York City in March, the researchers included another 719 people in their study who suspected they had Covid-19 based on symptoms and exposure to the virus, but in whom the illness had not been diagnosed.
In this group, the researchers found a different picture altogether. Most of these people — 62% — did not seem to have antibodies. Some may have been tested too soon after their illness for antibodies to be detectable. But many probably mistook influenza, another viral infection or even allergies for Covid-19, Dr Wajnberg said.
Another finding from the study — that diagnostic PCR tests can be positive up to 28 days after the start of infection — is also important, said Taia Wang, a viral immunologist at Stanford University. These tests look for genetic fragments, not antibodies, and suggest an active or waning infection.
“As far as known unknowns about Sars-CoV-2, this one really stands out,” she said. “We really need to know, how long does it take the body to clear the virus? How long are people contagious? We don’t know the answer to that.”
She and other scientists said it was unlikely that a positive test so long after symptoms appeared represents infectious virus. Researchers in South Korea recently announced, for example, that several suspected cases of “reinfection” were a result of PCR tests picking up remnants of dead virus.
Experts said the next step would be to confirm the presence of antibodies in the blood means protection from the coronavirus. The body depends on a subset of antibodies, called neutralising antibodies, to shield it from the coronavirus.
“The question now becomes to what extent those are neutralising antibodies and whether that leads to protection from infection — all of which we should presume are yes,” said Sean Whelan, a virologist at Washington University in St Louis.