Q: I have plans to fly to California and will have close contact with my unvaccinated grandchild. I received both doses of the Pfizer vaccine; however, my history includes B-cell lymphoma treated with rituximab (completed 2014). I’m concerned that my immune system may not have mounted a robust response to the vaccine and that I could become infected and transmit infection to my family. Should I have a blood test for COVID-19 antibodies?

A: That’s a great question. You were on rituximab, which “wipes out” B-cells for up to six months. The medication was used to help kill the cancerous B-cells that made up your lymphoma. B-cells are important because they’re what produce antibodies. Antibody levels can be measured after some vaccines to see if the person is protected. Unfortunately, the protective antibody levels aren’t known for COVID-19.

Many immunosuppressive medications weaken the response to vaccination. Because of this, vaccines should be administered prior to initiation of treatment whenever feasible. Of course, this isn’t possible with the seasonal influenza vaccine as it is given annually. Sometimes, vaccines can be administered at strategic times between immunosuppressive medication doses.

Even with immunosuppressive medications, COVID-19 vaccination may be able to protect against hospitalization and death even if it cannot completely block infection. It’s likely that vaccination gives the immune system enough of a head start to prevent the worst outcomes. Since you’ve been off the medication for years, you probably had a good immune response to the vaccine.

Regarding your question, there are limitations with checking antibodies after receiving a COVID-19 vaccine, and the U.S. Food and Drug Administration strongly recommends against it. First, the antibodies can be from either a prior undiagnosed COVID-19 infection or from vaccination. More importantly, the level of antibodies needed for protection isn’t known.

An absence of antibodies doesn’t necessarily mean a person isn’t protected as there’s another branch of protection known as cell-mediated immunity. Cell-mediated protection doesn’t depend on B-cells or their antibodies but rather T-cells and other cells that gobble up the virus and destroy infected cells. Theoretically, antibodies may not be needed for protection if cell-mediated immunity is strong enough.

Vaccinated people only produce antibodies against the spike protein. Routinely available tests don’t necessarily test for antibodies against the spike protein but rather may test for antibodies against other proteins the virus produces. This is another reason that a negative antibody test may not mean much. In other words, many people with normal immune systems test negative on some routinely available antibody tests.

No vaccine is 100 percent effective, and breakthrough infections may occur. We’re vaccinated to try to lead as normal lives as possible and may want to accept some risk associated with travel. If you want to play it extra safe, then wear a mask in public places and get a nasal COVID-19 test prior to departing to reduce the chance of spreading the virus to your family. Bon voyage!

Vaccine Smarts is written by Sealy Institute for Vaccine Sciences faculty members Drs. Megan Berman, an associate professor of internal medicine, and Richard Rupp, a professor of pediatrics at the University of Texas Medical Branch. For questions about vaccines, email vaccine.smarts@utmb.edu.

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