By Dr. Mallika Marshall

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.

READ MORE: Man In 'Grave Condition' After South Shore Plaza Shooting In Braintree; No Arrests Made Yet

Marie writes, “I know many people with the virus who still tested positive weeks later. So why now are we telling people they need a negative test to come out of isolation after five days?”

A PCR test that is performed in a lab can remain positive for weeks after infection. If you have been infected, you do not need to get a repeat PCR test to determine whether you can come out of isolation or return to work. It can remain positive long after you’re no longer infectious. However, a rapid antigen test will turn negative as you become less contagious. So if you’re infected but do an antigen test after five days and it’s negative, you’re probably no longer infectious and can come out of isolation, wearing a mask around others for another five days. If the antigen test is still positive, you need to stay in isolation for an additional five days.

An anonymous viewer wants to know, “Is the booster really necessary?”

READ MORE: Driver Killed By Commuter Rail Train In Wilmington Identified; MBTA Says 'Human Error' Focus Of Investigation

Yes, the booster is necessary. Many experts say we should think of the COVID mRNA vaccines as a series of three shots rather than two shots and a booster. With the most recent variants, the two-shot regimen continues to provide some protection but not nearly as much as with prior variants. Getting a booster will give you an added shield against severe illness and hospitalization.

Martha wants to know, “How effective are cloth masks with a PM2.5 filter?”

Wearing a cloth mask with a PM2.5 filter is certainly better than wearing a cloth mask alone. But wearing an N95 or KN95 mask is going to give you a better seal and likely better protection against Omicron, which is highly contagious.

Linda writes, “My 3-year-old grandson had COVID and just as he was about to emerge from quarantine, his 7-month-old sister tested positive. I was going to take my grandson during the day to help his parents, but should I worry about taking one while the other is still positive?”

MORE NEWS: Vigil And March Held In Belmont For Henry Tapia, One Year After Being Killed In Road Rage Confrontation

If it’s been at least 10 days since your grandson developed symptoms or tested positive and he’s feeling better, he is probably no longer contagious, even if his sister is now sick. He has probably built up some immunity that should protect him from getting infected again for at least several months. However, I would not be around his sister until she has come out of quarantine.

Dr. Mallika Marshall