New blood tests for the novel coronavirus could one day play a major role in showing how many people were previously infected with COVID-19, but experts caution that the new tests are not very good at telling us who has the disease now.

That’s because the blood tests don’t look for the virus itself, but for substances called antibodies that a patient’s immune system makes to fight off the virus. Once a person’s immune system recognizes a virus as a threat, it creates these antibodies to attach to the virus and neutralize it.

But that process takes time, and those antibodies often wouldn’t show up in a blood test until a week or more after the person started showing symptoms, according to Dr. Sonya Heath, a professor at UAB’s division of infectious diseases.

“When you're in a very early part of an infection, and in the case of COVID probably at least the first seven days of symptoms, you actually have to look for the virus itself,” Heath said. “You have to do the COVID PCR test, that's the nasal swab, and you're looking for the virus itself.”

Those nasal swabs collect mucus from deep within the patient’s nasal passages, where the virus tends to accumulate. That allows doctors to look for the virus itself rather than the antibodies.

“For COVID, the antibody response, or the immune response that your body makes in response to the COVID virus, comes up somewhere between day seven and day 14,” Heath said.

And that’s seven to 14 days after showing symptoms. The U.S. Centers for Disease Control now says that a patient may not show symptoms for two to 14 days after being infected. If the higher ends of those ranges are correct, a person who has COVID-19 may not test positive on a blood test for up to 28 days after getting infected.

Evidence also continues to show that people can transmit the COVID virus for several days before showing symptoms, and some people who test positive for the virus have reported very mild or no symptoms.

There’s also a risk of false positives with the blood tests, which have been rushed into use to combat this new virus. The novel coronavirus that causes COVID-19 is similar in structure to other coronaviruses, like the ones that cause the common cold.

Heath said there is a risk that these new tests would detect antibodies produced to fight the common cold or another virus and come back positive even if the patient hadn’t been exposed to the new virus that causes COVID-19.

Mobile tests may give incomplete picture

The long delay is one reason why the blood tests are not used to officially diagnose patients who believe they may have the virus. But in some cases, those blood tests are more easily available than the nasal swab.

In Mobile, approximately 900 police officers and fire fighters were given the blood tests in late March or early April because the nasal swab tests were not readily available at that time. Of those 900 personnel, 37 tested positive for the COVID antibodies, and received nasal swab tests. Three of those 37 tested positive with the nasal swabs.

Mobile Mayor Sandy Stimpson was optimistic when discussing those results at Tuesday’s city council meeting.

“If you think of only 3 of 900 had the virus, that’s 3 tenths of a percent,” Stimpson said. “You would have to assume our policemen and firemen are representative of our community. They probably come in contact with more people than others in the community.”

But it is unlikely that only 3 of the 900 had the virus. The 37 who tested positive for the antibodies may have had the virus previously and recovered from it before their nasal swab samples were taken.

And, more importantly, the emergency responders who did not test positive for the antibody may have already been infected but were just too early in their infection to register antibodies.

Testing negative for the antibody is not an “all-clear.”

“At this time, I don't think we're using an antibody test to necessarily declare that somebody can go back to work,” Heath said. “That's not a current recommendation from the CDC.”

Blood tests can lead to better understanding, treatments

Heath said blood antibody tests can provide valuable information after the fact, when trying to determine who has already had the virus and who has not. With many people unable to get tested in the early weeks of the disease, these blood test may clue researchers in later to just how many people were infected.

“We know that there are people who were infected with COVID in the early part of March in Alabama, but we didn't really have a test to look for that,” Heath said. “And so oftentimes, doctors were going based on most common symptoms that were compatible with what we understood about COVID.

“With the increasing availability of this antibody response, we will now have the ability to test people who may have had COVID, historically, sometime in the month of March.”

Heath said we don’t know how long those antibodies will last in the bloodstream, but it could potentially be months or years. It’s also still uncertain whether a person who has had COVID and recovered would still be at risk of getting the virus again.

“We all know that people get reinfected with common colds but we really don't know with coronavirus,” Heath said. “If we do find that people won't get reinfected then people may want to know if they've ever had it and if they have the antibody, because they'll have at least some sense that they've already been exposed and recovered from it.”

The antibody test may also be useful in developing a type of treatment using “convalescent plasma,” or blood plasma from a patient who has already recovered from COVID-19 and already has the antibodies. Those antibodies may help jump-start the immune response in a sick patient who has not yet developed the antibodies.

“UAB is working on understanding the antibody response, both whether or not people are generating antibodies, but also which of those antibodies can neutralize the virus,” Heath said. “If we can better understand that, we might be able to say a patient that's recovered from COVID has really good antibodies, and they could be a plasma donor, and that could be used in treatment for another person.”

AL.com reporter John Sharp contributed to this story.