By Dr. Mallika Marshall

BOSTON (CBS) – Dr. Mallika Marshall is answering your coronavirus-related medical questions. If you have a question for Dr. Mallika, 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.

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After one uses hand sanitizer, how long does the protection last? Can you safely go into a store and not worry about picking up germs as you touch things? David from Milford

Once hand sanitizer dries on your hands, it has done its job, killing most of the germs. But as soon as you touch something else like a doorknob or a dirty tissue, your hands can get contaminated again. So, you need to use more hand sanitizer every time you touch something that could be soiled.

Is it safe to dine indoors seated across from someone you do not live with? – Antonette

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I don’t think it’s a good idea to dine at an indoor restaurant, whether you’re with people you live with or not, especially if you’re at high risk. It’s hard to distance yourself from others inside, sitting at a table, and you have to take your masks off to eat and drink.

Cate has a question about COVID testing. She wonders why some people are just having to swab the inside of their nostrils while others have a swab placed deep in the back of the nasal cavity.

It depends on where you go to have testing. At our clinic, we are still using the deep swab which is more uncomfortable but may result in a better sample and therefore may be more accurate. There is another test which patients do themselves by swirling a cotton swab inside each nostril. It’s more comfortable and less risky for the health care provider but may not be quite as accurate.

The last and only flu shot I had was in the early 70s and I spent the next 36 hours in the emergency room with a severe reaction. Should I get one now? I’m 73 with no underlying health issues. – Wendy

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Having a severe reaction to the flu vaccine is rare, but you should talk to your doctor to determine whether it was a true reaction or an allergy. Flu vaccines have changed since the 1970s, but your doctor may want you to get a flu vaccine under close observation.

Dr. Mallika Marshall