The vaccines from Pfizer and Moderna require two doses, 21 and 28 days apart respectively. You need to get the same subsequent vaccine injection (either Pfizer or Moderna) each time. If you miss the exact date for your second injection, you should get it as soon as possible. You can expect your immunity to become active two or three weeks after the second injection.

The J&J/Janssen vaccine only requires a single dose, and you can expect your immunity to become active two or three weeks after that injection. Other COVID-19 vaccines are in development and will be available after they complete FDA’s review and authorization process.

Scientists are investigating how long immunity lasts and if people will need a booster in a year or so. While all three vaccines are highly effective at preventing serious illness from the virus, they should not be considered treatment if you currently have COVID-19. If you do contract the virus, you still need to see a medical provider to get proper treatment.

The Centers for Disease Control and Prevention (CDC) approved a set of recommendations from a panel of independent experts advising the agency on the rollout of the Pfizer, Moderna, and J&J/Janssen COVID-19 vaccines. While states are not required to follow the CDC’s recommendations, most probably will. Ultimately, state and local health departments will determine who will be eligible for vaccination in each phase.

Adults who are not in the tier 1 groups—healthy adults under 65 who don’t work in health care or otherwise qualify as essential workers—should have access to the vaccine by May.

Currently, no COVID-19 vaccine is authorized for use in children under the age of 16. Vaccine makers are currently conducting trials for teens and younger children.

While individuals who come to the emergency department or an urgent care center may be vaccinated (once it’s ready and available for their respective group), it may be easier to get vaccinated by your primary care provider, pharmacist, or local health department, particularly if they need to follow up with you about getting the second dose of the Pfizer or Moderna vaccines.

All of the available vaccines effectively protect against severe illness from COVID-19. The ability to choose which you receive depends on a number of factors, including the supply in your area at the time you’re vaccinated and whether certain vaccines are found to be more effective in certain populations, such as older adults. However, if you receive a Pfizer or Moderna vaccine, you need to get the same shot each time.

Yes, the vaccine is free, although you may want to check with your health insurer about whether they will cover the cost to administer the vaccine.

As with all vaccines, clinical trials rigorously evaluated the COVID-19 vaccine to generate scientific data and other information for the U.S. Food and Drug Administration (FDA) to determine their safety and effectiveness. Both Moderna and Pfizer's vaccines prevented more than 90 percent of COVID-19 cases, and even those that did contract the virus only had mild symptoms. The J&J/Janssen vaccine prevented 66.3 percent of COVID-19, and similarly protected against severe symptoms.

While "Project Warp Speed" removed the barriers and time associated with reporting and other logistical barriers, the companies still had to follow the FDA’s usual vaccine approval process to ensure they provide the appropriate protections and meet all required safety measures. Each company’s application to the FDA included two months of follow-up safety data from Phase 3 of clinical trials conducted by universities and other independent bodies. In that phase, tens of thousands of volunteers got a vaccine and waited to see if they became infected, compared with others who received a placebo. No serious safety concerns were reported in any of the vaccines.

Even after a vaccine is authorized or approved for use, there are vaccine safety monitoring systems that watch for adverse events or possible side effects. This continued monitoring can pick up on possible side effects that may not have been seen in clinical trials. If an unexpected adverse event is detected, experts quickly study it further to assess whether it is a true safety concern. Experts then decide whether changes are needed in the vaccine recommendations.

One of these systems is the CDC’s voluntary V-SAFE program. V-SAFE will use text messaging and web surveys for the CDC to check in with those who received the COVID-19 vaccine to see if health problems arose following vaccination. The system also will follow up via phone with anyone who reports medically significant side effects.

Some participants in vaccine trials experienced typical mild viral symptoms including fever, muscle aches, bad headaches, and fatigue after receiving the shots, but the side effects generally did not last more than a day. You may have stronger symptoms 24 hours after the second dose. Preliminary data suggests that, compared with most flu vaccines, the COVID-19 shots have a somewhat higher rate of such reactions, which are almost always normal signs that the body’s immune response is kicking in.

A small number of those first vaccinated had an allergic reaction following the injection. Therefore, we recommend that those with a history of significant allergic reactions or anaphylaxis check with their physicians before receiving the vaccine.

Researchers and government officials will continue to monitor those who have received the vaccine to study potential side-effects and determine if we need to adjust our current vaccine recommendations. Despite the chance of possible side effects, the vaccine is a safe and critically necessary step in ensuring we, as a society, can help to reduce the spread of the virus, end of this pandemic, and return to our usual way of life.

Yes, you should still plan on getting the vaccine. Although people who have contracted the virus do have immunity—called natural immunity—it is unclear how long it lasts. Some early evidence seems to suggest that natural immunity may not last very long. If you were treated for COVID-19 with monoclonal antibodies or convalescent plasma, you should wait 90 days before getting a COVID-19 vaccine.

The Pfizer, Moderna, and J&J/Janssen vaccines are very effective at preventing serious illness from the virus, however, it may be possible for those who have been vaccinated to get infected without developing symptoms and unknowingly transmit the virus to others.

Yes, once you get vaccinated you should still plan to wear a protective mask when in public and maintain social distance from unvaccinated people. However, fully-vaccinated people can visit with other fully vaccinated people indoors without wearing masks or physical distancing.

It takes several weeks for your body to build an adequate protective response, so you will need to continue pre-vaccine mitigation efforts during this transition time. Experts need to understand more about the protection that the COVID-19 vaccine provides before making a recommendation about when you no longer need to wear a mask and socially distance even after your immunity kicks in due to the silent transmission factor. Other elements, including how many people get vaccinated and how the virus is spreading in communities, will also affect this decision.

Experts do not know what percentage of people would need to get vaccinated to achieve herd immunity to COVID-19.

“Herd immunity” is a term used to describe when enough people have protection—either from previous infection or vaccination—that it is unlikely a virus or bacteria can spread and cause disease. As a result, everyone within the community is protected even if some people don’t have any protection themselves. The percentage of people who need to have protection in order to achieve herd immunity varies by disease.

You can visit the CDC’s COVID-19 vaccine page or visit the website of your state’s department of health if you have additional questions.