BOSTON (CBS) — Dr. Mallika Marshall is answering your coronavirus vaccine-related medical questions. If you have a question, email her or message her on Facebook or Twitter.

Dr. Mallika is offering her best advice, but as always, consult your personal doctor before making any decisions about your personal health.

Toni writes, “I got the J&J shot. I’m wondering if I can also get an mRNA shot now too – just to be extra protected? I’m so upset with myself for getting the J&J shot instead of the Moderna I was originally scheduled to get. I’m hoping I can rectify the situation now.”

Don’t be upset. The J&J vaccine is a perfectly good vaccine and will protect you from severe disease, which is the ultimate goal. My own husband got the J&J vaccine! It is possible, however, that you will be advised to get a booster shot, either with the same vaccine or with one of the mRNA vaccines, for added protection against the delta variant. Guidance on this will likely come in the near future. Stay tuned.

Jim writes, “I had my second Moderna shot in February. In June, I was tested for Covid-19 antibodies. This turned out negative. What should I do? Is a third shot available?”

An antibody test is not an accurate way to determine whether you have developed immunity to the coronavirus through vaccination. In fact, it’s likely to be negative even if the vaccine is working. That’s because many of the standard coronavirus antibody tests detect parts of the virus that are not targeted by the vaccines. Unless you are immunocompromised, you should assume that you have mounted a good response to the vaccines.

Josephine asks, “Is it safe to get the shot if I have seizures?”

Yes. Always check with your primary care physician about your concerns but people with seizure disorders can get the COVID vaccines.

Michael writes, “Greetings from Senegal. In March, I received the first dose of the AstraZeneca vaccine but there are availability issues and it does not look like I can get the second dose. I have plans to travel back to MA in the next few weeks. Should I get two jabs of the Pfizer vaccine?

It is probably safe to mix vaccines but we’re still waiting for more data before formal recommendations are made. I would check with your provider, but if you can’t get a second AstraZeneca dose, you probably should get a second dose of another vaccine. And if you get Pfizer, you probably only need one dose.

Dr. Mallika Marshall