TORONTO -- It’s a term you've likely heard before: herd immunity.

"Fundamentally, you have to get the disease from somebody and if everyone around you is immunized then fewer people can give you the disease and so you’re indirectly protected and that’s what herd immunity is," CTV News’ science and technology specialist Dan Riskin said.

Medical experts believe COVID-19 can be kept under control once enough people are immunized and they believe safest way to reach herd immunity is through mass vaccinations.

To determine how many people need to be vaccinated to reach herd immunity, two variables are needed. 

‘R0’ (pronounced “R naught”), which is the reproduction number or how much a disease can spread if there are no restrictions, and ‘E’ the efficacy rate, how well the vaccine is working in a controlled setting.

The formula is more nuanced than it seems, as each variable can fluctuate. 

For example, the reproduction number — how many people get sick from one infected person on average — can depend on a number of factors, including location.

We’ve reached herd immunity before against the measles, but when it comes to COVID-19, some medical experts aren’t convinced we can do it again.

“We might have a lot of vaccinations in one geographic area, but not enough in another and so you can’t get herd immunity if you have pockets of people who are vaccinated or not,” said Jane Heffernan, an Associate Professor in the Department of Mathematics and Statistics at York University.

She and Riskin also agree it’s unclear whether asymptomatic people can spread the disease.

“We also don’t really have a very good knowledge of how many people have been actually infected asymptomatically with this virus, so there needs to be more studies,” Heffernan said.

It’s also unclear how long a person will be protected from each vaccine.

“The good news is it doesn’t have to be a one-shotted vaccine and that was our chance and we missed it. If it didn’t work, we can keep using vaccines as different strains emerge,” Riskin said.

Herd immunity is dependent on the number of people who choose to get vaccinated, however there’s no consensus among experts on exactly how many people that needs to be.