DEAR DR. ROACH: My daughter was recently visiting and tested positive for COVID. Since my last COVID vaccine was eight months ago, I searched for how much protection I still had and understood the response to be little to none. So, why is the recommendation for vaccination only once per year and not more frequent so that we always have some protection?

— B.J.M.

ANSWER: The mRNA COVID vaccines provide 85% to 90% protection for the first three months or so, but the immunity wanes to only 50% to 60% effectiveness against getting the disease after six to eight months. The vaccine efficacy depends on the particular vaccine and precise variant of COVID. So, you are right that the vaccine you got eight months ago provides little protection against getting the disease now.

However, the vaccine also protects people against severe diseases and the currently circulating variants of COVID, even if it can’t protect you as well in getting the disease at all. The likelihood of hospitalization and death is decreased for much longer after vaccination, especially if a person has had multiple vaccines. This protection extends for a year and possibly longer.

Getting vaccines twice yearly is not harmful and would provide more consistent (but still not perfect) protection, but once yearly vaccination does protect against severe diseases, including hospitalization and death.

People with immune systems that don’t work perfectly, such as the elderly, and people who are on medicines that affect the immune system will not respond as well to the vaccine. So, they may particularly benefit from more frequent vaccination.

The decision to get vaccinated should be up to a patient and their physician.

DEAR DR. ROACH: A beloved cousin of mine was given the wrong type of blood as a teenager and subsequently died. I, myself, am type O Rh-negative. What are the odds of this happening today, and how can this be prevented?

— R.P.

ANSWER: I am very sorry about your cousin. This is an extraordinarily rare event, with four definite cases occurring in the United States (out of 10 million transfusions) in the most recently reported year. It should never happen; there are supposed to be multiple layers of protection to keep this from occuring. A person who is O-negative, like you, is at the highest risk for a transfusion reaction as you can only safely get O-negative blood.

Blood is absolutely tested before every transfusion with limited exceptions. In a true emergency, sometimes O-negative blood is given without crossmatching. This reduces the risk of severe transfusion reactions, but there are other blood antigens beyond the ABO system.