Dr. Aaron Glatt, chair of the Department of Medicine at Mount Sinai South Nassau hospital, spoke to Newsday on Thursday about how mass vaccinations for COVID-19 can lead to herd immunity. Credit: Newsday / Steve Pfost/Steve Pfost

Health care officials frustrated by a shortage of COVID-19 vaccines may face the opposite problem in a few months: trying to convince enough Americans to get vaccinated to reach herd immunity.

No one knows what percentage of the public must be vaccinated to achieve herd immunity, but experts’ estimates generally range between 70% and 90%. Herd immunity would be reached when enough of the population has antibodies to the virus so that it no longer spreads widely, although there still would be some cases.

"Our goal is to reach that herd immunity as quickly as we can, both nationally and globally," said Dr. Uzma Syed, an infectious disease specialist at Good Samaritan Hospital in West Islip. "That’s the only way we’ll get this behind us. It’s essentially a race of us against this pandemic. And with more and more variants discovered, the more the urgency there is in getting people vaccinated."

Medical experts fear more contagious variants of the virus may complicate efforts to reach herd immunity, and achieving it depends on other unknowns — such as how effective vaccines are in preventing transmission and how many Americans will remain reluctant to take the vaccine.

If mutated variants spread widely and are more contagious and more resistant to vaccines, "That may significantly move the goalpost," said Dr. Bruce Farber, chief of infectious diseases at North Shore University Hospital in Manhasset and Long Island Jewish Medical Center in New Hyde Park.

"The better vaccines you have, the lower the percentage of the population you need to have vaccinated for population immunity," said Dr. Wafaa El-Sadr, a professor of epidemiology and medicine at Columbia University in Manhattan.

Although the vaccines are highly effective in preventing serious disease, it’s still unknown how effective they are in stopping virus transmission, El-Sadr said.

A paper released Feb. 1, but not yet peer-reviewed, found that the AstraZeneca/University of Oxford vaccine — which may go before the U.S. Food and Drug Administration for authorization within the next few weeks — led to a 67% drop in positive coronavirus test results after one dose. That, Oxford researchers said in the paper, indicates the vaccine "may have a substantial impact on transmission by reducing the number of infected individuals in the population."

The data is encouraging, said Dr. Aaron Glatt, chairman of medicine and chief of infectious diseases at Mount Sinai South Nassau hospital in Oceanside. Even for those who do test positive after taking that vaccine or others, such as the Pfizer/BioNTech or Moderna ones, their ability to transmit the virus may be lower, he said.

The vaccines have been shown to prevent sickness and, in those who do become ill, mitigate the severity of sickness. The amount of virus in the bodies of people who are asymptomatic or mildly sick is believed to be lower than in those who are seriously ill, so it’s believed they are less likely to infect others, he said.

Dr. Anthony Fauci, President Joe Biden’s chief medical adviser on COVID-19 and director of the National Institute of Allergy and Infectious Diseases, said last week that herd immunity could be reached as early as late summer or early fall, depending on variants and other factors.

Yet don’t expect life to get back to pre-COVID-19 normal, said Farber, a member of the state’s clinical advisory task force, which reviews vaccines.

"I don’t think COVID is ever going to completely disappear," he said.

But, he said, herd immunity would mean "we dramatically decrease transmissions, people feel safer, their risks are significantly lower."

For example, with herd immunity, there would still be a small risk of contracting the virus at a crowded event like a concert. But if the person is vaccinated and does get sick, the illness would probably be relatively mild, Farber said.

He said there are some similarities between COVID-19 under herd immunity and the flu, which is deadly for a relatively small minority, causes severe illness in others and leads to non-life-threatening sickness for the large majority — but is far less dangerous than COVID-19 is today.

Polls in December and January showed that roughly 30% of the U.S. public does not plan to get vaccinated, although acceptance of the coronavirus vaccines has risen over the past few months.

Another obstacle to reaching herd immunity: It likely will be months until kids under 16 will be eligible for vaccines, perhaps not until 2022 for younger children, experts say.

El-Sadr said not enough has been done to work with trusted community leaders to address the reasons for vaccine hesitancy.

"Many of them are not anti-vaccine," she said of those reluctant to get vaccinated. "Most of them have legitimate questions," such as why the vaccines are safe if they were developed so quickly and whether vaccines were tested in people like them.

There also should be public-service advertisements featuring elected officials and celebrities, who "are huge influencers" of behavior, said Perry Halkitis, dean of the Rutgers School of Public Health.

He pointed to how Elvis Presley receiving the polio vaccine before going on national television in 1956 helped boost vaccination rates among young people. The vaccine led to a sharp drop in polio cases and its U.S. eradication in 1979.

Glatt said he’s concerned about vaccine hesitancy and the widespread misinformation on social media. But he is confident herd immunity will be reached.

"Hopefully people will see that the people who get vaccinated aren’t getting sick and the people who keep on getting sick are those who aren’t vaccinated," he said. "Hopefully that will change their opinion."

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