Just how many Americans need to be vaccinated before we stop the runaway transmission of COVID-19? That is the $64 million question that experts cannot accurately answer because so many factors are at play.

In December, Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, estimated we need at least 75% to 85% of our citizens vaccinated to achieve herd immunity.

According to Fast Company, that threshold means enough people will be immunized to indirectly protect those who do not get vaccinated. But with record-breaking cases and deaths, experts say we need a more reasonable and achievable target to slow the rate of transmission of COVID-19.

Dr. William Schaffner, an expert in infectious diseases at Vanderbilt University Medical Center, estimates it will require inoculating more than 50% of the population to trigger a downtick of the disease. So far, the Centers for Disease Control and Prevention says about 9 million have received their first dose, representing 2.7% of the country. And we cannot count on achieving herd immunity from those who have had COVID-19 as estimates for that cohort are about 20% to 25% of the U.S. population.

Dr. Keri Althoff, associate professor of epidemiology at the Johns Hopkins School of Public Health, tells Fast Company that when essential workers who are out and about in the community get their shots, we may see a downturn in the number of infections.

"Those individuals becoming vaccinated, and building up a higher level of population immunity, will likely have a bigger impact in immunity," she said.

She added, when older people get vaccinated the death rate might also go down, even though the elderly are not likely to spread COVID-19 because they tend to isolate themselves.

"So, it's about who you are vaccinating and when," Althoff said, "as to whether or not you will see specifically deaths go down, as to whether or not you will see specifically infections go down."

Another factor to consider are the new variants being identified that might drastically change the thresholds for herd immunity. A recent British study confirmed what researchers have been telling us about the coronavirus variant that has invaded the U.K. and now the U.S. The study found the variant is so contagious that drastic measures might have to be implemented to contain the spread of disease.

According to The New York Times, Dr. Nicholas Davies, a prominent epidemiologist who led the study, said the mutation is approximately 50% more transmissible than the original virus. Other studies have shown one of the reasons the new variant is so virulent is it infects human cells more easily than the original, according to NPR.

"The more contagious the variant, the higher proportion of the population you have to vaccinate in order to reduce transmission," said Schaffner. Experts say, while vaccinations are a critical part of containing the virus, we still need a multifaceted public health strategy that includes wearing masks, social distancing, and washing hands, says Fast Company.

"Those are not things we throw by the wayside at all right now," Althoff said. "Vaccination is a tool we're adding to our toolbox."