Wondering how effective the COVID-19 vaccines are or how long immunity lasts? This medical expert and investigator on the vaccines' clinical trials has the answers you need.

With vaccinations to tackle the coronavirus (COVID-19) pandemic well underway, your mind may be swirling with dozens of burning Qs about the various vaccines and their effects. That's why Purvi Parikh, M.D. — an allergist and immunologist with the Allergy & Asthma Network who worked on the Pfizer and AstraZeneca vaccine trials at NYU Langone — is here to clear up any confusion.

How do these vaccines work?

"The Pfizer and Moderna vaccines use a new technology known as messenger RNA. It’s like a cheat sheet of instructions, a blueprint that essentially tricks your cells into thinking they’ve been infected with COVID-19. It does this by mimicking a part of the virus called the spike, or S protein, which is believed to cause the infection by binding to our cells. That in turn gets your immune system to mount a response.

To be clear: You are not being injected with the virus. Your cells just think you’ve been injected with it. The AstraZeneca vaccine, on the other hand, uses inactivated cold viruses loaded with COVID-19 genes to prompt your immune system to respond. [Note: The Food and Drug Administration hasn't yet approved the AstraZeneca vaccine for use in the U.S.] With all three vaccines, you will need two injections about one month apart. It takes two doses to achieve the strong immune response that stops the virus."

See: How Effective Is the COVID-19 Vaccine?

Are they safe?

"Yes, they area. But there’s so much misinformation out there. The message I want to send is this: Trust the scientific process. The National Institutes of Health and the FDA are doing their due diligence. I see examples of that on a daily basis. You can be assured that the same rigorous scientific process used to create any vaccine has been followed with the COVID-19 vaccines, along with all the proper safety checks and balances." (FYI, you can look to this guide for a general idea of when you'll get the vaccine.)

What about possible side effects?

"The most common have been fever, fatigue, muscle aches, and chills. Some people had muscle aches and pain at the injection site in their arm. But not everyone gets them, and for those who do, all the side effects have resolved within a few days."

Read More: What to Know About COVID-19 Vaccine Side Effects

How long will immunity last after you get vaccinated?

"That is the million-dollar question. We don’t know. What we do know is that people who have gotten sick with COVID-19 still have good immunity four, five, even six months later. That’s a positive sign. But on the flip side, we are also seeing cases of reinfection, as well as the loss of antibodies. While rare, the reinfections have been severe in some cases.

The immunity part of it is complicated. We hear a lot about antibodies, but there’s another critical component of your immune system called T cells, which help target and destroy cells in the body infected by viruses. A lot of the vaccine trials are looking at antibody immunity and T cell immunity — because if you have both, that really improves your chances of not getting sick."

How many people need to be vaccinated to achieve herd immunity?

"Most epidemiologists and virologists say that 70 to 80 percent of the population needs to get the vaccine for that to happen. People say, 'Why don’t we just let everybody get sick and reach herd immunity that way?' But that would harm many people because some COVID-19 patients become very ill and die. Even for those who survive, there can be long-term effects. Some are still struggling eight months later with blood clots, scarred lungs, or neurological issues. And it’s not just older adults — it’s a lot of young, previously healthy people too.

Remember, the vaccine is all we have. There is no other defense against this virus besides social distancing and wearing masks. If people don’t get the vaccine, we’ll never get to herd immunity."