Covid-19: Risk after Vaccination, Masks, and CDC Missteps

First, a definition. Vaccine effectiveness is defined as the proportionate reduction in cases in a vaccinated population. There are three words I want to focus on:

1. Effectiveness. This is a semantic quibble but I think an important one. We are talking about effectiveness which refers to “real-world” conditions, not efficacy which would refer to clinical trial conditions. This is important because “real-world” conditions are more representative of your world (which I assume is real).
2. Proportionate. This means that we take one number and reduce it by a percentage.
3. Vaccinated population. All we can do is estimate risk across a population of individuals and assume our individual risk is similar to that seen over the population.

To do the math we need the current COVID-19 infection rate and then we reduce that number by the estimated vaccine effectiveness. What? You don’t have those numbers?? Well lucky for you, I do.

Over the past seven days, the COVID-19 infection rate is about 0.8% for the entire U.S., yet this is not terribly representative. As I’ve said before, bagels in New York taste different than bagels in Wyoming, meaning that some areas will be higher than 0.8% and others will be lower. But for the sake of argument, let’s assume that number is accurate.

We know that the effectiveness of the mRNA vaccines (Moderna and Pfizer) in preventing symptomatic COVID-19 infection is at least 90%. That number does NOT mean that 90% of those vaccinated don’t get sick, nor does it mean that 10% do get sick. That would actually not be nearly as good as our new reality. No, using the definition of vaccine effectiveness above, that means if the baseline infection rate in the population is 0.8%, then you reduce that 0.8% by 90%. A 90% reduction of 0.8% means that the infection rate goes from 0.8% down to 0.08%. That equates to a population risk and, assuming each individual has exactly the population risk, of a one in 1,250 risk of symptomatic COVID-19 infection.




Now before this post goes viral (if only) and an epidemiologist scolds me, I know that I’m technically using the wrong terms. And yes, there is a big disclaimer: we can’t say for certain what an individual’s precise risk of COVID-19 infection is because it depends on multiple variables such as mask wearing, who the person is around and for how long and in what environment, community rate of infection, etc.

My point in using these numbers is not to say that any one of us has exactly a one in 1,250 risk of symptomatic COVID-19 infection. Rather it is to try and express in numerical terms just how low the risk of infection is if you are fully vaccinated with a mRNA vaccine.

What is a person’s risk of hospitalization or death from COVID-19 infection if they have been fully vaccinated?

The short answer: really, really low. Why? Because only a minority of patients with COVID-19 get hospitalized or die. Let’s take a worst-case scenario. Let’s hypothetically assume that the infection fatality ratio for everyone in the population is about 9%, which may be a reasonable estimate for only the highest risk individuals (e.g. the elderly). That is 9% of all infections (including asymptomatic infections that never get tested) resulting in death. Again, that isn’t a real number, this is simply to prove a point.

If our infection rate is 0.8% of the population and only 9% of that group die of the infection, then our population fatality rate is 0.072%. That number is pretty low, but wait… the number gets lower because we have to reduce that very low number by 90%! So the mortality rate in the population at large is about 0.007% — a population risk of one in 14,286 people.




Again, I’m not saying these numbers are highly precise risk estimates. They are reasonable illustrations, however, of just how low the risk of death or hospitalization from COVID-19 is in people who are fully vaccinated.

But wait, I’ve heard about breakthrough infections. What are those all about?

A 90% vaccine effectiveness rate is not 100%. Breakthrough infections are bound to happen. They happen with every other vaccine because nothing is 100% effective. In fact, the rule of large numbers I’ve discussed previously equally applies here. Even a very small percentage of a large number will yield a large number. We are bound to hear about breakthrough infections not because the vaccines are ineffective, but because the number of vaccinated people is so large.

How frequent are breakthrough infections, and how bad are they?

Again, the easy abstract answers: very infrequent and not bad. But let’s put some details to those answers to make the point.

First, a regional real-world example. The Cleveland Clinic reported that from January through April 99.7% of the infections they diagnosed — and 99.7% of the hospitalizations in their institution due to COVID-19 — occurred in people who were not fully vaccinated. Let that sink in. In a major health system, fully vaccinated individuals accounted for only 0.3% of infections and hospitalizations.

Second, a national real-world example. As of May 10, the CDC reported 115 million fully vaccinated people in the U.S. At the same time, the CDC knew of 794 hospitalizations due to COVID-19 and 181 COVID-19 related deaths in those 115 million fully vaccinated people. I strongly doubt those numbers of hospitalizations or deaths are a significant undercount because of how vigorously COVID-19 is still looked for and reported. But for the sake of argument, let’s double those numbers, so there would be 1,588 hospitalizations and 362 deaths among those 115 million fully vaccinated persons. That means 0.001% of all fully vaccinated persons have been hospitalized with COVID-19 and 0.0003% of all fully vaccinated persons have died from COVID-19; and I think those are overestimates.

All the descriptive reports I’ve read (I don’t know of firm data on this point) make it clear that breakthrough infections, in addition to being rare, are often more mild than infections in non-vaccinated individuals. This makes perfect sense because your vaccinated immune system is still generating a response that lessens the severity of infection, even if it doesn’t prevent infection outright.

But can fully vaccinated individuals pose a risk to others?

In the first month or two after the EUAs for the Moderna and Pfizer vaccines there was still some question about whether these vaccines prevented infection as well as disease (symptomatic infection). There is no longer any doubt — if you are fully vaccinated the chances of you getting infected with COVID-19 are so low that you do not pose a risk of spreading the virus to other people.




And dismiss concerns of “carrying” the virus. As I’ve pointed out before (but can’t remember in which post — a sign I’ve written too much or not enough), humans don’t “carry” respiratory viruses in the way normally thought of. People can have asymptomatic infection, but that only lasts 10 days or so. Besides, all that is moot because we now have evidence that COVID-19 vaccines prevent infections and not just disease.

What does all this mean for people who are vaccinated?

Most obviously it means that fully vaccinated individuals should consider themselves well protected against COVID-19. In addition, fully vaccinated people should be comfortable that they do not pose a COVID-19 threat to others.

I will not tell anyone how to feel (I have enough problems with my own feelings). This pandemic has been a traumatizing experience for everyone, obviously far more so for some than others. As such, misgivings about resuming “normal” interactions are normal and ok.

At the same time (notice this is not a “but”) it’s important to mix those feelings with the cold, hard logic that these vaccines, especially the mRNA vaccines, have done their job. That job was never to eliminate the risk of COVID-19. Such a goal was always unrealistic. The goal was to lower our individual risk from COVID-19 to be no higher than any other risk we accepted (even if we didn’t know we were accepting such risks) before the pandemic. These vaccines have made the risk of COVID-19 for fully vaccinated individuals much lower than that for other infectious diseases and for the most common health risks such as heart disease, cancer, etc.

What do you think of the CDC statement that it’s safe for fully vaccinated individuals to go “maskless”?

I’ve been reading CDC publications and guidance statements for over two decades. The recent guidance, like most other CDC statements, was well-intentioned, scientifically sound, and written by hard-working and underappreciated professionals. Unfortunately, as is also par for the course, the timing and method of release was poorly thought out, and the implications for the public not at all appreciated. My experience is the CDC knows health well, the public health extraordinarily well, but the public at large, not so much.

Wow, it sounds like you have something to get off your chest. Please go on.

Thank you. I will.

There was absolutely no preparing the public for this about face. From news reports, it appears that no one outside the CDC even knew it was coming. That’s a problem because a month ago the director of the CDC was tearfully stating she had a sense of impending doom. I made the case both here and here that there was little reason for doom and much more reason for optimism. Besides that, every message I know of from the CDC since then has been one of “Yes the vaccines are good, but we’re so far away from normal.”




After 16 months of “Mask up or you are putting everyone else at terrible risk,” how did CDC officials think this guidance would be interpreted? The CDC director said on “Meet the Press” on Sunday, “This was not permission to shed masks for everybody everywhere.”

True, the words themselves didn’t say that, but the context and the timing of those words given to an exhausted public absolutely gave that impression. Unwinding mask mandates was always going to be messy and haphazard, like most things in a decentralized, federalist republic that jealously guards its individual liberty. And yes, the CDC has been under immense political and public pressure. I just wish there had been more awareness of the public and yes, some more competence in the timing, wording, and preparation for the guidance.

End of my rant. Let me end on a high note.

I have listened to nearly every public FDA and CDC committee hearing on the COVID-19 vaccines. Consistently people are missing the amazing forest because they’re focused on their own tree. We are in the middle of a monumental achievement and there is much reason for gratitude and optimism.

Stay safe, and go make some lemonade.

Dr. Adam Jackson was an Infectious Diseases clinical pharmacy specialist in Denver for 19 years. He received his PharmD from the University of Florida in 1998 and then completed an Infectious Diseases Pharmacy Practice residency at Bay Pines VA Medical Center in St. Petersburg, Florida in 1998-99. While in his previous role, he served the Infectious Diseases team reviewing the pharmacotherapy needs of their nearly 1,000 patients with HIV including antiretroviral management and cardiac risk assessment. In addition, he served as a liaison between the Infectious Diseases department and Primary Care as well as other specialty departments in optimizing the antibiotic choices for patients and developing regional guidelines on the treatment of community-acquired infectious diseases. Adam also worked on regional immunization guidelines, initiatives, and communicating these recommendations. Adam now works in Drug Utilization Management. He writes at The Infectious Pharmacist, from where this piece is reprinted with permission.