When Will We Ever Reach Herd Immunity?

Originally Published By Katelyn Jetelina

To stop this pandemic, we need at least 70% (231M) people to have long, lasting antibodies to protect against COVID19 disease. Period.

How do we get there? Well, it’s really quite a simple equation: Herd immunity= “# of naturally infected that have lasting antibodies” + “# already with full dosage of vaccines” + “# with half doses of vaccines” + “# people we need interested in getting the vaccine.” Here we are with each one of these numbers…

“of naturally infected that have lasting antibodies”

Until now, we didn’t truly know how many people were “naturally” infected with COVID19. It was estimated to range between 2-10 times the number of reported cases. A new study reported that the number of COVID19 cases in the US is nearly 3 times that of confirmed cases. This means more than 71 million (26.7%) people in the United States contracted COVID19 (which is higher than the reported 26.7 million cases). This is helpful (when thinking about herd immunity). While we don’t know how many of these 71 million have lasting, strong antibodies, we can start making some educated guesses. Other scientific studies have estimated that at LEAST 10% of people “naturally” infected have lasting b-cell antibodies (I’ve posted about this before). This is a gross underestimate because we are still learning about COVID19 T-cells and immune response after infection. So, 10% of people (low end) would be 7.1 million people that can help us reach herd immunity. So, the absolute low end of this would be 3%; the absolute higher end would be 20% towards herd immunity. We will take what we can get.

“already with full dosage of vaccines”

Today, there are 2.9% of Americans with both doses in their arms. For these rough models, we will assume 100% effectiveness (which it’s not).

“already with half dosage of vaccines”

Today, there are 6.8% of Americans with one dose in their arm. For these rough models, we will assume 100% effectiveness (which it’s not).

“people we need to get vaccinated”

Good news is that people are becoming LESS hesitant. CDC published a study today that found “intent to vaccinate” increased from 39.4% (in September) to 49.1% (in December) AND non-intent decreased from 38.1% (in September) to 32.1% (in December). Kaiser research group (called Kaiser Family Foundation COVID19 Vaccine Monitor) came out with similar numbers:  The people who want to get vaccinated as soon as possible increased from 34% (in December) to 41% (in January).

So, today, the math adds up to 3-20% (natural infection) + 2.9% (2 doses in arms) + 6.8% (1 dose in arms) = 12.7% (low end)-29.7% (high end) herd immunity. If everyone who wants a vaccine could get one today, herd immunity would increase to 53.7% (low end). Which is good, but not enough.

Reaching herd immunity (and WHEN we end this pandemic) hinges on three things:

1) The amount of protection from natural infection, which we are still trying to figure out (so get your vaccine even if you have COVID19)

2) Convincing people that the vaccine is safe, effective, and the only step towards herd immunity

3) The impact new variants have on the herd immunity threshold. I based today’s post on 70% herd immunity, but many scientists are now estimating that this is closer to 90% with the new variants.

Your Local Epidemiologist, Katelyn Jetelina