Omission bias is the phenomenon in which people prefer omission (inaction) over commission (action) and tend to think that harm as a result of inaction is much more acceptable than harm from action.

One example of omission bias is the fact that some people prefer not to vaccinate their child because if one extremely rare complication of a vaccine happens, they will judge it much more harshly than if a much more frequent and severe complication from the virus infection itself happens to their unvaccinated child. [1]

In other words, if an unvaccinated child dies from a preventable infection, the parent will accept that much more than if the vaccinated child has a rare and benign reaction to a vaccine.

Let’s take measles for a more precise example:

The average American probably doesn’t remember what measles is and how dangerous it can be. As a result, some people will prefer omission (not getting the measles vaccine) over commission (getting the measles vaccine).

But here are 10 things you probably didn’t know about measles:

1. Measles is a preventable viral disease, and thanks to the MMR vaccine, measles was declared eliminated from the US in 2000, but is making a resurgence with over 884 cases (out of those, 11 percent have needed hospitalization) and three deaths in the US so far in 2025. [2]
2. Prior to the measles vaccine in 1966, we had three to four million cases of measles per year, around 48,000 hospitalizations, 1,000 cases of chronic encephalitis (brain inflammation), and 500 confirmed deaths due to measles in the US every year. [3] [4]
3. In order to have our community protected (called herd immunity) against measles, we need at least 95 percent of our population to be vaccinated, which is a number that was reached in 2000, but a number that is not reached anymore in the US. [5]
4. The measles virus is extremely contagious, with the infectivity starting 4 days before the rash, which means that somebody could be contagious even before knowing they have measles. Each infected individual could infect 9 out of 10 unvaccinated people around him or her, and the virus can stay in the air up to two hours after the infected person has left the room.[6]
5. Measles incubation is long, about 7 to 14 days [7], which means that somebody exposed to measles could be vaccinated at the time of infection and be protected against the disease.
6. The measles virus can infect any cell in our body: our lungs causing pneumonia, our cardiovascular system, our eyes (conjunctivitis), our ears (otitis media because of obstruction of Eustachian tubes), our skin (descending rash that starts with the face then descends to the rest of the body), our spinal cord (myelitis) and our brain (encephalitis) to just name a few systems. Dr. Jay Varma explains in his thorough article for PsychologyToday the different ways our brain can be damaged by the measles virus. [8]
7. The measles virus can cause immune amnesia for two to three years after infection, which means it can cause immune suppression and make individuals more susceptible to multiple unrelated infections for two to three years. [9]
8. Measles can give complications not only at the time of infection but also 7 to 10 years after the infection: One of those complications is called subacute sclerotic panencephalitis which is a neurodegenerative disease that is often fatal and for which there is no cure. Subacute sclerotic panencephalitis is due to persistence of the virus in the brain, and it can progress in four stages. After a period ranging from one month to 27 years without symptoms. Stage one may be mood swings, depression or personality changes that can last up to 6 months, stage two may be muscle spasms and seizures, stage three often is twisting movements and rigidity, stage four is paralysis and progressive loss of consciousness, often followed by death. [10]
9. There is no treatment for measles. Some people recommend vitamin A but because vitamin A isn’t easily excreted, vitamin A has led to liver damage in children. [11]
10. The measles vaccine (MMR) is very safe, possibly giving per 10,000 people vaccinated three fever-related seizures, 0.4 abnormal blood clotting, and 0.035 allergic reactions, numbers that are minimal compared to the potential complications of the measles infection itself. [12]

So, because there are so many potential complications from measles, much more than the few possible complications from the vaccine, let’s be aware of our omission bias and let’s vaccinate our children.