For example, someone who is pregnant or has a weakened immune system should not get the MMR vaccine. It's possible there will be certain conditions for any COVID-19 vaccine as well. These people will have to rely on herd immunity (ideally attained mostly through vaccinations) to ensure they stay healthy. If someone is eligible to get a vaccine but chooses not to, that would be putting others at risk.
Vaccines are also not 100% effective, some people who get vaccinated are not themselves fully immune. If you choose not to vaccinate you also put those people at risk.
Finally, if you choose not vaccinate and end up in the ICU, you are taking resources in a critical ward that might be needed for patients with other serious medical conditions.
Non-deliberate immunosuppression can occur in, for example, ataxia–telangiectasia, complement deficiencies, many types of cancer, and certain chronic infections such as human immunodeficiency virus (HIV).It varies by vaccine, but allergies, health conditions, and age are all factors.
Some people also practice religions that forbid them.
What are the ethics, though, of refusing to get a vaccine? if Alex Honnold had fallen off the face of El Capitan, he would have killed only himself. Is it not the same in the case of someone refusing to be vaccinated, given that the vaccination is available to everyone who does want it?
There is one difference, I suppose. The person who refuses vaccination and subsequently gets a serious case of COVID-19 causes society to spend a good deal of money treating them in the hospital. (Even if they have insurance they are needlessly increasing the costs of care for everyone.)