An epidemiologist at the University of Alabama at Birmingham projected that the country, and Alabama, could reach herd immunity to COVID-19 as early as May or June.
“We’re getting closer to herd immunity, thanks to the vaccine, and also new scientific data that shows us that more people had COVID than were tested,” said Suzanne Judd, a PhD epidemiologist at UAB.
Judd said her calculations are based on the current pace of delivering COVID-19 vaccines, coupled with a recent study from Columbia University that estimated that more than a third of the U.S. population may have already been infected with the virus by the end of January.
Herd immunity occurs when the virus cannot spread through a population uncontrolled because enough members of the population have some kind of immunity, either from a vaccine or from antibodies if they had the disease and recovered.
Scientists already know how many people have been vaccinated against COVID and how many people have tested positive for the disease. But there’s a huge, and unknown, number of people who got the virus and were never tested.
If that number is large enough, spread of the disease could drop dramatically in the coming months.
“Current estimates are that we’ll see [herd immunity] sometime late spring, early summer in Alabama,” Judd said. “Somewhere between May and June is likely, but this depends on many factors.”
Judd said new studies at Columbia, Johns Hopkins University and UAB suggest that many more people may have already had the disease than we thought. In New York City, she said, one study showed a nearly 10-to-1 ratio of people who currently have antibodies to people who tested positive for the virus.
Judd said that’s less surprising because New York was hit hard by the virus early in the pandemic, when it was much harder to get a COVID test. She said at UAB, the ratio is closer to 5-to-1. Her forecast, in an attempt be conservative and not over-estimate, assumes there are three people who actually had COVID in Alabama for every positive test reported.
“The more people that have immunity, the less the virus will spread, the safer it will be for us to interact with one another again,” Judd said.
Estimates vary on how much of the population needs to be immune to truly disrupt the virus’s spread, but Judd said her calculations were based on a threshold of getting immunity in 72% of the population.
“There’s no magical number that is herd immunity,” Judd said. “Each virus, each bacteria is different, and they mutate at regular intervals, so this number can move. But at the moment, scientists are shooting for about 72% of the population with immunity, so hopefully we’ll see that soon.”
For Alabama, Judd said that would mean 3.5 million people need to be either vaccinated or infected. Current statistics show that 12.5% of Alabama’s population had received at least one COVID dose of vaccine and another 10% had tested positive for the virus.
As of Friday, Alabama had reported 491,849 positive tests. Assuming a 3-to-1 ratio, that would indicate that almost 1.5 million Alabamians have had COVID already. If the 5-to-1 ratio turns out to be true, that would equal more than 2.4 million people have already been infected, bringing the state closer to that herd immunity threshold.
Alabama State Health Officer Dr. Scott Harris said he was familiar with Judd’s work and said she did a “tremendous job” forecasting herd immunity for Alabama.
“Dr. Judd is a great researcher, and so I think that what she put together was very intriguing and makes a lot of sense,” Harris said Thursday.
Still, Harris said there were many unknowns about herd immunity, including how many Alabamians may have already gotten COVID but never got tested.
“A little over 10% or so of our state we know has tested positive, and has been infected,” Harris said. “But certainly the percentage that has antibodies on board is a lot higher than that.”
Mutations also could be key to whether herd immunity holds up, she said, or if booster vaccines are needed.
“It really depends on how the virus mutates,” she said. “There are some mutations that we’ll be completely covered by the immune response the body has already developed. There are other mutations that could escape the immune system and require a vaccine booster or actually lead to reinfection.
“So the variants are something we have to watch very carefully.”
Judd said Alabamians should not let their guard down, or begin leaving their masks at home just yet.
“We’re not going to know that we’re at herd immunity, there’s no magical, all of a sudden we’re at 72% and we’re safe,” Judd said. “The things we’re going to monitor are the number of cases per week, we’re going to continue to watch that very carefully. And as long as it trends down and stays below about 10 or 5 [cases] per 100,000, and we have decent testing in the state, then we’ll know that it’s safe to get people back together.”
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