Feb. 12, 2021 -- It could take almost 9 more months to vaccinate 70% of Americans against coronavirus infection and reach herd immunity, assuming the current pace of immunizations continues and requires a two-dose regimen with either the Pfizer/BioNTech or Moderna vaccines, a Medscape analysis found.

Health care workers have administered 44,769,970 doses of COVID-19 vaccine in the United States as of Feb. 11, according to the CDC’s COVID Data Tracker. Although the site also reports total U.S. cases of COVID-19, total doses of vaccines delivered, and the number of Americans who have died, the agency doesn't estimate how long it might take to reach certain immunization milestones.

To predict the proportion of the American population likely to be immunized over time, Medscape decided to do the math. An initial assumption is that vaccinations will continue at 1.44 million doses per day, the highest moving 7-day average as of Feb. 10.

Although figures vary, scientists estimate that between 70% and 85% of Americans will need to be vaccinated against the coronavirus to achieve herd immunity. The Medscape analysis is based on the more optimistic 70% figure.

A Moving Target

One of the major caveats of the analysis is that the number of vaccinations is "hard to predict, as the situation changes day by day," Ana Pastore y Piontti, PhD, associate research scientist at the Laboratory for the Modeling of Biological and Sociotechnical Systems at Northeastern University in Boston, told Medscape. "We don't know what the daily doses could be in the months to come."

CDC Director Rochelle Walensky, MD, said the agency is working toward President Joe Biden’s goal of 100 million doses available within 100 days, which will be April 30. However, it seems unlikely that COVID-19 vaccine access will expand to all Americans by the end of this month or March, as predicted by previous administration officials. "We are in an encouraging period of time with vaccines rolling out," Joshua Barocas, MD, assistant professor of medicine at the Boston University School of Medicine, said during a Infectious Diseases Society of America COVID-19 update for journalists Feb. 3). However, "remember that only a very small portion of the population has been vaccinated" so mask wearing, handwashing, social distancing, and other measures remain essential.

Exactly when the US population could reach herd immunity is "the million-dollar question," Ricardo Franco, MD, assistant professor of medicine at the University of Alabama at Birmingham, said during the IDSA media briefing.

Assuming you have a 70% herd immunity threshold and ideal vaccination programs, "you can do the math. But include vaccine hesitancy and some variants that may still circulate, [and] it’s a hard question," he said.

Picking Up the Pace

The Medscape analysis is intended to provide an estimated timeline for herd immunity. The calculations will likely need to be adjusted going forward. For example, if and when single-dose COVID-19 vaccines gain emergency use authorization from the FDA, the number of available doses could jump substantially.

The FDA is expected to announce its review of the data for the Janssen/Johnson & Johnson single-dose application on Feb. 26.

On the other hand, the modeling assumes no vaccine hesitancy among U.S. residents. If reluctance to receive the vaccines increases, the timeline could even stretch into 2023. But, if educational efforts and other factors persuade more people to get immunized, the timeline could shorten.

Coronavirus variants, any one of which could require a redesigned "booster" vaccination in the future, also could throw a wrench into any modeling.

"The variants change the equation in at least two ways," said Barocas. "If the vaccines have less efficacy against some of the variants, it definitely pushes that timeline out further."

It's also important to focus on getting the vaccine rollout into diverse communities and vulnerable populations, he said."The sooner we can get vaccines into arms, the less important the variants become," Barocas said.

Also, if vaccine distribution programs become more efficient — if federal, state, and local governments provide more funding and other resources to accelerate the rate of immunizations — more Americans would likely be immunized sooner.

"It is important to quickly report the number of vaccine shots given to the CDC, so we know which locations have the greatest need for more vials," Franco said.