Will You Definitely Get COVID If You Don’t Get Vaccinated?

Do You Feel Lucky?

F. Perry Wilson, MD MSCE


Last week, I had an interesting conversation with a friend of mine from college. Like many docs these days, I find myself doing my best to answer a lot of COVID-related questions from friends and family — ranging from straightforward stuff to edge cases that never seem to quite be covered by the CDC recommendations.

But this one led to a question that I’ve actually been stuck on for a bit — and I want to share my thinking with you.

COVID-19 Mortality risk by age group

My friend has not yet gotten vaccinated. He is not really anti-vaccine. He falls into that more common category of wait and see — nervous about some reported side-effects, in no rush to get the jab. I try to meet people where they are when it comes to this — not overly dogmatic — just trying to honestly outline the risks and benefits. Like me, he’s a forty-something guy without a high-risk medical condition. He understood that, if he were to get COVID, there’s a good chance it would be mild, a small chance it would be severe, and a very small chance it would lead to death. Fine.

But then he said something I hadn’t thought of. He asked — and what’s the chance that I’ll get COVID at all?

That question had me stuck for a second. I mean, it has to be less than 100% right? Nothing in medicine is 100%.

So here’s my best effort to answer the question. If you are not vaccinated, will you get COVID-19?

The first thing we need to think about is the much vaunted herd immunity. For the people in the back, this is the concept that when a certain proportion of a population is immune to a disease, either through infection or vaccination, the epidemic essentially burns itself out. In other words, the “herd” protects those individuals without immunity. They may never get sick.

The herd immunity threshold is tied directly to the basic reproduction number of the pathogen — the average number of people each infected person subsequently infects in a totally vulnerable population.



F. Perry Wilson, MD MSCE

Medicine, science, statistics. Associate Professor of Medicine and Public Health at Yale. New book “How Medicine Works and When it Doesn’t” available now.