As coronavirus vaccines continue to be administered in California, people in the community are sending in questions to our newsroom.KCRA 3's Brittany Johnson has been tracking all of the questions and spoke with Dr. Stuart Cohen, Chief of Infectious Diseases at UC Davis Health, to get some answers.Q: Can you test positive for COVID-19 if you've already received a coronavirus vaccine?Dr. Cohen: "The answer to that is probably yes but not 100% certainty. Let me try to explain that in a little bit more detail. We know that the vaccine prevents the disease from occurring in 95% of people, which means 5% could actually still get the disease. What we don't know is whether the vaccine actually totally prevents infection, or whether it just prevents disease. So, you will be able on a blood test to determine that you have developed antibodies, which is the immunity that the vaccine provides you. But on a nasal swab or a nasal pharyngeal swab, it is possible that you could still have coronavirus present in your nasal secretions. Those data will eventually be available and we'll be able to answer that question more definitively. But the point right now is I cannot guarantee that it won't prevent, that it will totally prevent the infection and therefore that you could still be positive."Q: Are coronavirus vaccines going to be like flu shots that we have to get yearly?Dr. Cohen: "I'm thinking that it will be something like that. Maybe it'll be every two years or every three years. I think as the virus changes, the vaccine is going to have to change. I anticipate that it will be a repeated vaccine, just not sure how frequently."Q: If I don't get the flu vaccine can I still get a COVID-19 vaccine?Dr. Cohen: "The answer to that is yes."Q: If a person has had COVID-19 and recovered, are they already immune? Will their immunity be short term? How long do you anticipate the vaccines to be effective?Dr. Cohen: "Those are a couple of different questions sort of mixed in. If somebody has COVID-19 they are immune. The data at this point suggests that people may be at least immune out to eight months. Will that wane over time? Most likely. Will that be less effective as the virus changes? Most likely. So we still tell people that if they've had COVID-19, they should get vaccinated because that will boost their immune system and maybe cause the immunity to last longer. We just want people to wait probably three months after they have been diagnosed, or more so that they avoid getting a more severe reaction to the vaccine."Q: What are the side effects for people who have health issues already and putting this experimental drug in our bodies?Dr. Cohen: "Let me make a couple of points about that. First, the two vaccines that are authorized for emergency use are really not experimental drugs anymore. They are not fully FDA approved because they haven't gone through the entire two year follow up period that is necessary for final FDA approval, but they have been tested, and this is no longer an experiment. We know that the vaccines work. They both have been tested in over 30,000 people. They both are about 95% effective. People who have underlying illnesses, there are no real data that suggests that you're more likely or less likely to get side effects if you get the vaccine. So most of the side effects that people get from the vaccine are local; people get sore arms, I suspect probably almost everybody does, and people might get swelling in the arm, they may get some other local changes, symptoms that are more systemic, like getting fever or things like that are much less likely. There's no data to suggest that if you have an underlying illness, you're more likely to get the response."Q: Which of the vaccines do you prefer? Dr. Cohen: "I think they're both very, very similar. I would just say whichever one you can get, you should take. I don't think there's enough different about them to really make a decision like this."Q: Should I hold out for a certain vaccine of my choice?Dr. Cohen: "The Moderna vaccine is one of the vaccines that's available so that it has the same efficacy rate or effectiveness rate as the Pfizer vaccine, which is about 95%. The advantage of the Moderna vaccine is more of an advantage of delivery. So you can store the maternal vaccine and a regular freezer, as opposed to having a minus 70 and ultra-low temperature freezer that you have to store the Pfizer vaccine. But they're very similar in the way that they've been made, and they're very similar in the way that they work. I think as far as waiting -- so obviously, every person has their own free choice to decide that. But the pandemic is happening now. And particularly now, when the number of cases are spiking up, this is the time when you really would benefit from having the vaccine -- even if you're going to use a mask, and you're going to maintain your social distancing, and limit the number of people that you interact with, and try to do your interactions outdoors ... When people have talked to me about this before, when I talk to my own patients, the big picture is that I think everyone is getting a little tired of this pandemic, to say the least. I think if we're trying to figure out what's the best way out of this, vaccinating people is the best way out of this for people who are older like me. All my friends and myself as well, we all got measles. We all got chickenpox. We all got mumps. We all got diseases that our kids don't get because they've got the vaccines. That's the, that's the big picture. This is a way to contain this pandemic."Q: What happens if you mix manufacturers and get one vaccine from Pfizer and the other from Moderna?Dr. Cohen: "That's an interesting question. We do our best not to do that. I don't know what exactly would be the situation. I don't think I would dissuade somebody from mixing vaccines if that was the only way they could get their second dose, but I think the right way to do it and the optimal strategy is to get both shots with the same vaccine."Q: Do coronavirus vaccines carry the virus? If so, once getting it do you become a carrier?Dr. Cohen: "The vaccine contains some genetic material from the virus called RNA. That is the genetic material that's responsible for making one protein. One part of the coronavirus is called the spike protein. That spike protein is what our body reacts to, to try to contain the infection. That spike protein is what the virus uses to attach to ourselves to infect you. Once the vaccine is injected, the RNA gets into your cells, it makes some of these proteins that make some spike protein, and then the RNA is destroyed by the cells. So that's it. There is no active virus, there is no replication. People don't become carriers. It's just a different way of delivering the target of our immune response."Q: Is the vaccine recommended for women who are pregnant or for women who are planning to conceive?Dr. Cohen: "The questions that you're asking are really good because they're not perfectly answerable. Here's the best answer I can give you. Pregnant women are excluded from all the clinical trials that have been done with these vaccines. What we know from vaccinating women with other long-standing vaccines is that as long as the vaccine does not contain a live organism, whether it's a bacteria or a virus, it is safe. So, we are suggesting that pregnant women be vaccinated and that they would get benefit from receiving the vaccine. I would say that until we've vaccinated a number of pregnant women we will not know for absolute certainty that it's safe. But based on all of our other experiences with other vaccines, for example, flu vaccine, we suggest that pregnant women get vaccinated for influenza. This is a different vaccine, but it's a similar type of thing. There's nothing live in the vaccine. We just want to protect mom and protect the baby."Q: How can we get herd immunity if we don't know how long the vaccines will be effective for?Dr. Cohen: "I will say that it's time-sensitive, right? I understand that if you don't know how long the vaccine is going to last, herd immunity may not last forever. That's why you may need to be revaccinated. But what you have to do right now, is you have to have enough people infected or vaccinated or immune to keep people from getting infected further ... So here's the way that herd immunity is going to work. So right now, if you have a population of people who have never been either infected with COVID-19 or received vaccine, for every person that's infected, there will be two to three more people who are infected, and then those two to three will infect another two to three, and the numbers get big relatively quickly as you can picture. The way to stop this is for the one person who's infected to bump into almost nobody who can be infected because they're already immune and they've been vaccinated. And then the number of cases start dropping. If it takes 15 people to infect another person, then you can see those numbers are going to start dropping. And so even if the immunity wanes over time, that's why we may have to revaccinate people. But for the most part, it can get the pandemic initially under control, and get it down to a number that seems reasonable -- where every time you walk into the grocery store, you don't have to be fearful that you're going to bump into somebody who's potentially infectious."Q: Do Pfizer and Moderna vaccines protect against different strains of coronavirus?Dr. Cohen: "There are some laboratory data, particularly with the Pfizer vaccine right now, that looks at the changes associated with the new strains and demonstrates that the antibodies do protect against those changes within that strain. So, the assumption is going to be yes, they do work. There will need to be a little bit more data than what we have right now to confirm that that's true, but at least preliminary data suggests that they will protect against the new strains."