Getting to herd immunity will require 90% of people to be vaccinated against COVID-19, experts say

• The rise of the more contagious Delta variant means that even more of us need to get vaccinated.
• An expert formula suggests 90% of the population needs to be vaccinated to stop the spread of COVID-19.
• "We will get to a point where we're comfortable," one expert said, but only if 25-30% more people get shots.

Our 'when will the pandemic end' equation has changed.

This is not a figure of speech. There really is a formula experts use to calculate what it will take to stop the spread of a vaccine-preventable illness like COVID-19.

That percentage - a so-called 'community protection' threshold - is going way, way up.

Contagiousness and vaccine effectiveness drive the equation

Taken from the go-to physicians' manual for shots, Plotkin's Vaccines, the equation is simple, and driven by two factors. One is the contagiousness of a disease. This is what scientists refer to as R0 (R-naught) and it measures how quickly the disease can spread.

Because the Delta variant is about twice as contagious as other versions of the virus, it more easily spreads from person to person. So, R0 is up.

Vaccine expert Paul Offit from Children's Hospital of Philadelphia, a co-author of the Plotkin's vaccine handbook, uses an R0 somewhere between five and seven for Delta, similar to Centers for Disease Control and Prevention estimates. (An R0 of 7 means one infected person could reasonably be expected to infect about seven other people.)

The second factor to consider in our immunity equation is how well a given immunization works at beating the virus.

"Obviously, the more effective the vaccine, the fewer people you would need to vaccinate," Offit said.

That's part of the reason disease experts were thrilled when some of the first vaccines authorized during the pandemic turned out to be around 95% effective at preventing illness. But those blockbuster vaccine effectiveness numbers have started to come down, in part because vaccine-induced immunity wanes over time, but mainly because more evasive variants like Delta arrived before enough people were vaccinated to stop them, just as everyone started taking off their masks.

Offit says a rough and "generous" estimate for our current vaccine effectiveness, given these recent developments, is around 90%.

90% of us need to be vaccinated to kill off Delta

The Plotkin's textbook formula for community immunity through vaccination involves a bit of long division:

[(R0 - 1)/ R0] / % vaccine efficacy

With Delta now responsible for essentially all of the COVID-19 infections in the US, that's:

[(7-1)/7]/90% = 95%

Or, assuming a slightly lower R0 of 5: [(5-1)/5]/90% = 90%

Any way you do this math with Delta, around nine out of ten of us need to get vaccinated in order to achieve a level of widespread disease protection. Other leading experts agree with the calculation, putting the figure for herd immunity well above 80%.

Even though some Americans who aren't vaccinated have had prior infections, which confers some immunity, "you really still need to have at least another 25% to 30% of people who have not been naturally infected or immunized to be immunized. You need that," Offit said.

Plus, "there is just a certain group of people who are not going to get vaccinated," he added. "What do you do then? I think the answer is what we're doing, which is mandate vaccines."

If we get somewhere near 90% vaccination in the US, we'll have the same level of protection from COVID-19 that we already do for many other vaccine-preventable diseases, like measles, polio, chickenpox, and tetanus.

"Then you'll see, I think, a pretty dramatic decline in the incidence of the disease," Offit said. "I mean, we're not going to eliminate it from this country, but we will get to a point where we're comfortable that the incidence of cases and deaths is low enough that we don't feel we need to change our life anymore."