What would it take to reach herd immunity in the U.S.?
"The simple straightforward estimate says 60-70% of the population," would have to become infected, said Dr. Saad Omer, an infectious disease expert and Director of the Yale Institute for Global Health. "But there are other estimates that say that because we as humans do not mix randomly and some people are more exposed earlier on and then develop immunity, et cetera, because their profession and their social habits, we may reach ‘herd immunity’ at somewhat lower levels."
Dr. Omer said the U.S. could start seeing infections slow down after even 40-45% of the population have been infected, depending on how the population is interacting.
To compare it with a wildfire, the fire can only burn where there is still fuel.
The more people that have developed immunity, the fewer people that can spread the virus. So there would be a noticeable impact even before a theoretical threshold is reached.
Dr. Omer added there is no magical number of infections where once you cross it, the virus stops spreading.
"Things start getting better before that," he said. "But they continue even after you achieve the herd immunity threshold."
That is because not all communities interact with one another, so there will continue to be pockets of the population that are vulnerable to outbreak. This can be seen with the measles, where skepticism of the measles vaccine has been growing among some parents for years, leading to outbreaks in communities with lower vaccination rates.
But ultimately, herd immunity is not something to strive for unless it is achieved through vaccination.
"The cost of that is a substantial number of people dying, and a substantial number of people as we increasingly know having long-term effects," said Dr. Omer.
About 40% of the U.S. population amounts to more than 130 million people.
If that many people were to get sick, many will not survive.
Public health experts have stressed that the key is to prevent infections from occurring in the first place, so that the virus is not circulating at a high level within the community.
When a vaccine becomes available, Dr. Omer said it can be deployed strategically to high-risk populations first, such as the elderly or essential workers, in order to slow down infections more quickly.