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.

Penny says, “My husband and I, both aged 70, are fully vaccinated. We wonder if it is safe for us to return to the gym. We canceled our membership last year due to COVID.”

Beginning May 29 many gyms will no longer require fully vaccinated staff and clients to wear masks. If you are immunocompromised, I would discourage you from returning to the gym. But if you’re otherwise healthy I think you need to weigh the risks and benefits. Obviously returning to a regular fitness routine is important for your physical and mental health. But you’ll be taking a small risk that you could be exposed to the virus indoors. For some, that is a risk worth taking. For others, it is not. If you decide to wait, try to stay active outdoors.

Dan writes, “Could someone get a two-shot regimen of Pfizer now to get increased protection after being vaccinated two months ago with J&J?”

It is not necessary for you to get re-immunized at this time. I know there have been some high-profile breakthrough cases with the Johnson & Johnson vaccine, but most of those people have been asymptomatic. You still are well protected against severe COVID-19 which is great. And there is some evidence that immunity continues to build with the J&J vaccine as time goes by, reaching more than 90-percent efficacy against severe COVID-19 at 56 days.

Dana asks, “I had been babysitting for a 9-year-old and the family would like me to come back on date nights. They are not vaccinated. Is it safe for me to go once a week to babysit with a mask on?”

If you’re otherwise healthy and fully vaccinated and the child is otherwise healthy, it should be relatively safe for you to babysit her without a mask even if she is unvaccinated. You should be well protected from the virus. And while there is a small risk that you could pass the virus on to her or her family, it’s not likely.

A concerned mom writes, “My daughter is 23 and had COVID in January. She got vaccinated with Moderna a few weeks ago. She says she regrets it because she read it could compromise childbearing. True?”

There is a false rumor circulating on social media that the COVID vaccines can cause infertility. This is NOT true, and it makes me sad to think that young women continue to believe this. There is no evidence that the vaccines affect a woman’s fertility. In fact, women got pregnant during the initial vaccine trials. Please tell your daughter not to worry. She did the right thing by getting vaccinated.

Tess writes, “My almost 13-year-old daughter is petite and only 64 lbs. She really wants to get the COVID vaccine but I am hesitant due to her small stature. Should I be concerned? Could her immune system go into overdrive?”

I understand your concern but vaccines are not dosed based on weight or size. That’s because vaccines generally do not enter the bloodstream but do their work right around the injection site by introducing just enough material to stimulate the body to mount an immune response. Sometimes vaccine doses are different for certain ages but sometimes they aren’t, even for babies. And in some cases, adults are actually given smaller doses of a particular vaccine than kids. It all depends.

Dr. Mallika Marshall