A: When we get a vaccine and lots of people have received it; or else, a very long time from now.
Even though we have well over 30 million confirmed cases and nearly a million deaths from COVID-19 worldwide, we’re not even close to reaching herd immunity. In fact, we’re not even close to the lowest estimates for herd immunity in the places that were absolutely slammed by COVID-19 early on, like Italy and Queens, NY.
Most scientists think that we need around 60-70% of people to be immune to COVID-19 in order to reach herd immunity. Herd immunity happens when a virus is starved of new hosts because susceptible people (those who are not immune) are so rare in the population that an infected person would probably not come into contact with a susceptible person.
We can get that herd immunity threshold a little bit lower if we’re really strategic about who is immune. For example, for pertussis (also known as whooping cough), you may have noticed that recent public health campaigns focus on vaccinating grandparents. Vaccinating grandparents is important because immunity from the pertussis vaccine starts to wear off as we age. We’re protecting newborns by making sure the people around them are immune. This strategy is called cocooning. There are other strategic ways to get the effective herd immunity threshold to be a little lower, such as requiring nursing home employees to be vaccinated annually for influenza. This breaks the chains of infection from school-aged children to their parents who are nursing home staff to nursing home residents.
But even with a strategic vaccination program, we probably can’t get the herd immunity threshold a lot lower than 60%.
We’ve been through the looking glass for 9 months, and so far about 6.9 million people in the United States have confirmed infections. That’s about equal to the total population of the state of Arizona.
Now let’s do some imagining. Let’s assume that having been previously infected actually does make us immune for life (which is still an open question). Let’s also assume that we’ve under-identified cases–in fact we’ve only identified 1 in every 5 cases. This is probably about right in the hardest-hit areas, and a wildly generous overestimate everywhere else.
That means we only have another 136 million cases to go until we reach herd immunity! 136 million people is roughly the combined population of the 10 biggest cities in the United States (New York + Los Angeles + Chicago + Houston + Phoenix + Philadelphia + San Antonio + San Diego + Dallas + San Jose) PLUS the entire population of Florida, Ohio, Georgia, North Carolina, Michigan, New Jersey, Virginia, Washington, Arizona, and Tennessee. Combined. This is fun, right?
We would also expect between 500,000 and 2 million deaths, depending on how good we are at protecting our most vulnerable from infection. For comparison, the number of deaths would be something like the total population of New Hampshire.
Now, let’s say that we’re all willing to accept the *completely insane* number of deaths involved in getting to herd immunity through natural infection. (Which, for the record, we are not. That is *not* a good public health strategy.)
At the current rate, we’ll reach herd immunity in… 14 years and 5 months!
Okay, you’re saying… so what if we accelerated the curve and just *get this over with*?
That would be very, very bad. If we sped up the process somehow and strategically infected almost half of the United States population with COVID-19 before New Years, based on hospital capacity constraints and underlying death rates, we would expect to see deaths totaling somewhere around the entire population of Texas. Everyone in our second-most populous state. Seriously.
When and if an effective vaccine becomes available, we’ll still need to vaccinate millions and millions of people. But when we get vaccinated, we become immune *without getting sick*. We can skip the part where half of us need to be infected and many hundreds of thousands of us die, and get straight to the immunity part.
Because of this cruel math and other reasons, the only reasonable way to achieve herd immunity is through a vaccine that is effective and widely adopted.
** This post was updated at 10am on Sept 24 to allow for more generous assumptions about cases never-identified for the rough model we present. That is to say, we adjusted the estimates to include the concept that even more people are already immune. **
For our previous post on herd immunity, check out.
And here’s another one: https://fivethirtyeight.com/…/without-a-vaccine-herd-immun…/