How soon to herd immunity as doses rise?
AUSTIN — With more than 7.4 million Texans having received at least one dose of a coronavirus vaccine and millions more previously infected and carrying antibodies, herd immunity could be on the horizon.
But how soon is anybody’s guess.
Estimates vary on what percent of the population must be protected from the virus to reach herd immunity, the point at which each infected person transmits the disease to an average of fewer than one other person, and it starts to die out.
Dr. Anthony Fauci, the nation’s top infectious disease expert, has said the number could be as high as 85 percent.
Dr. John Zerwas, a top medical adviser to Gov. Greg Abbott and vice chancellor for the University of Texas Health System, told the American-Statesman that even if the goal is 70 percent, “we’ve got a ways to go.”
Zerwas attributes the decline in cases and hospitalizations, two key indicators of the pandemic’s intensity, in part to the rise in vaccinations. Other health experts aren’t too sure.
“I definitely attribute (the decline in cases) to the combined effect of immunity from natural disease and vaccination,” Zerwas said, adding that older, higher risk residents have received a vaccine at even higher rates. More than six in 10 Texas seniors have received at least one dose, according to the latest data from the Texas Department of State Health Services.
“My sense is the impact of the current level of vaccinations or immunity that we have in that at-risk population is definitely something that’s contributing to the improvement in the numbers.”
Up to 13 million Texans may have natural immunity through previously having been infected, Zerwas said, a number he based on CDC guidance. The true number of infected people is thought to be many times higher than what’s been reported through testing. More than 2.4 million people in
Texas have tested positive for the coronavirus
“We can take a pretty good bit of confidence that the natural immunity is good protection immunity against the disease,” Zerwas said. “If you combine that with the number of people who have gotten some amount of vaccine, plus those that are naturally immune at this point, you get up to a pretty high rate.”
Hundreds of people with appointments wait in line to receive a COVID-19 vaccine at Gregory Gymnasium at the University of Texas on March 1. Starting last Monday, all Texans 16 and older are eligible to be vaccinated.
But Texas also has a large youth population, which could affect how quickly the state reaches herd immunity.
Zerwas estimated that 21 million people in Texas are 18 or older, making them eligible for all three authorized COVID-19 vaccines. The Pfizer vaccine is the only COVID-19 vaccine available for those 16 and older.
People who have previously contracted the virus should still get a coronavirus vaccine, according to federal health guidance.
“In the strictest sense, you have to take the whole population of Texas,” to determine herd immunity, Zerwas said, adding that between 8 to 9 million Texans are under 18. “It’s hard to say, but I like to tell people that I think the summer will be a lot more normal. By the end of April, I think we’re going to see a lot more normalcy to our lives.”
Even as some states are seeing an uptick in cases amid emerging variants and as national health experts warn of a fourth wave, case numbers in Texas continue to ebb.
It’s a massive shift from the picture three months ago, when hospitalizations and case numbers were rising quickly and intensive care beds were in short supply across the state.
Since a pandemic peak in midJanuary, cases have dropped to below 3,000 per day, levels not seen since early June. The seven-day rolling positivity rate also has hovered around 5.5 percent in recent days, the lowest in a year.
Still, some health experts say vaccinations aren’t the primary driver in the decline in cases and hospitalizations.
Jungsik Noh, an assistant professor at UT Southwestern in the Lyda Hill Department of Bioinformatics, estimated that as of Wednesday, 25 percent of Texans have been infected with the virus. Noh, who has studied the undercounting of COVID-19 cases, said his estimate does not consider vaccinations and is based on daily reported cases and deaths.
“I don’t think that the decrease is mainly driven by increasing vaccination rates and/or the number of people who have previously had COVID,” Noh said. “The vaccination rates and cumulative incidence rates are definitely the factors that contribute to slowing the spread, however their magnitude is not enough at this point.”
Noh also estimated that the number of Texans who “have some kind of immunity from previous infection or vaccination” is around 44.5 percent.
“In my opinion, no one has a clear idea of the herd immunity threshold,” Noh said, adding “44.5 percent seems to be still not enough in my view.”
Lauren Ancel Meyers, director of the UT COVID-19 modeling consortium, attributed the drop to changes in behaviors and policies at the local level.
“I don’t think we can yet attribute it definitively to vaccination,” she said. “We will eventually see the impact of vaccines. At some point we will really get to the point where even if we relaxed measures, transmission levels will be low, but we are just not there yet.”
That’s because, in part, some areas of Texas and some communities have higher vaccination rates than others, Meyers said.
Meyers’ team recently studied the potential impacts of spring break gatherings and more contagious coronavirus variants as Abbott ended statewide coronavirus restrictions, including the mask mandate.
The projections found that if the face mask order remained in place and travel remained low through spring break, cases would have declined even further than they have. In Austin, cases were on a rapid decline but recently reached a plateau, which Meyers said was a sign that Texas is not doing as well as it could be.
“It also is evidence that our behavior is still really driving transmission, and we’re still quite a ways away from the vaccines being sufficient to bring the virus under control,” Meyers said.