Since the beginning of the pandemic, scientists, physicians and public leaders have been talking about achieving herd immunity. But what exactly is herd immunity and why is it so important that we achieve it?
Put simply, it occurs when enough of the population has gained immunity against a given virus so that the spread of the disease is slowed, if not stopped completely. To fully understand herd immunity, it’s helpful to understand how viruses spread.
If you’ve ever been around a sick person, you’ve probably wondered, “Am I going to get sick?” The answer depends. Every virus has what’s called a basic reproduction number, or the number of people likely to get sick from typical contact with an infected person. For example, influenza’s reproduction number is usually between 1 and 2 and varies based on the yearly strains. This means that someone with the flu is likely to infect one to two people.
The basic reproduction number for the current coronavirus, SARS-CoV-2, is still not clear, given that it is a relatively new virus, but estimates range from 2 to 4. This means an infected person will likely cause two to four other people to get sick, and this is exactly what we saw in the early days of the pandemic; people who were infected, with or without symptoms, were encountering others and infecting them. These infected people encountered even more people and the virus multiplied two to four times with each contact.
Stopping this rapid spread is where herd immunity comes in. The unchecked spread of a virus only occurs if there is no immunity to it. The beauty of our immune systems is that once someone recovers from an infection, they have a level of immunity to reinfection. They are part of the immune herd and are much less likely to get infected.
With increasing numbers of immune people in a community, the virus spread will slow. If a COVID-19 infected person encounters a group with, say, four people and two of them are immune, the disease will not spread as fast because two out of the four people are much less likely to catch the disease, and thus won’t spread it to others.
In this scenario, however, the two people without immunity remain vulnerable to getting sick and spreading the disease. The key is to minimize the numbers of those at risk of infection so the “herd” is safe.
Using what we know about the infection rate and reproduction number for each virus, scientists can calculate what portion of our population needs to be immune in order to reach this safe threshold. For COVID-19, we need to reach about 85% of the population being immune to have sufficient herd immunity to slow or stop the spread.
There are only two ways to become immune. First, you can have — and recover from — COVID-19. This is a risky option because the disease can cause serious illness and death, and even those who recover may have long-lasting health effects.
The second — much safer — option is to be vaccinated. Given the potential danger of a COVID-19 infection, the urgent need to reach herd immunity as soon as possible, and the proven safety and efficacy of the available vaccines, getting a COVID-19 shot is the safest, most effective path to herd immunity, and a return to the normal life we all miss so much.
“Vaccination of as many people as possible, as soon as possible, is the safest and quickest path out of this pandemic,” says Hilary M. Babcock, MD, infectious disease specialist at Washington University and BJC HealthCare.