The available 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.

It is expected that in the coming months additional vaccines from other companies will be available and only require a single dose.

Scientists are investigating how long immunity lasts and if people will need a booster in a year or so.

While both the Pfizer and Moderna 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 and the Moderna COVID-19 vaccine. 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.

Health care workers—including emergency physicians, nurses, and others on the frontlines of battling the pandemic—will be eligible first, along with elderly people living in nursing homes and other long-term care facilities. The vaccine will be available for this group in mid- to late-December.

Next will likely be essential workers—those who work in food and agriculture, manufacturing, law enforcement, education, transportation, corrections, emergency response and other sectors. These individuals are at increased risk of exposure to the virus often because their jobs preclude them from working from home. These workers also are disproportionately Black and Hispanic—populations that have been hit especially hard by the virus.

After essential workers, the next groups recommended by the CDC committee are adults with medical conditions that put them at high risk of infection and people over the age of 65.

Keep in mind that while these are national recommendations, each state may have slightly different determinations about who falls into each category. Also, states don’t need to reach everyone in tier 1 group before moving on to the next, according to the CDC advisory committee.

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 or June, depending on the volume of the nation’s supply. By late spring/early summer, we expect two more vaccines (from Johnson & Johnson and Astra Zeneca) to be available.

At this point, neither the Pfizer nor Moderna vaccine have been tested in children so currently the vaccine is only available for adults. Likewise, the clinical trials did not include pregnant women. So those who are pregnant or breast feeding should talk with their physician about getting vaccinated.

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 for your primary care provider, pharmacist, or local health department to follow up with you about getting the second dose.

This 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. At first, the only choices will be the Pfizer and Moderna vaccine. You need to get the same shot (either Pfizer or Moderna) each time.

By late spring/early summer, we expect two more vaccines from Johnson & Johnson and Astra Zeneca to be available.

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 vaccines prevented more than 90 percent of COVID-19 cases, and even those that did contract the virus only had mild 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 both Pfizer’s and Moderna’s 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, but you should wait until those with a greater health risk receive theirs first. 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. Regarding vaccination, we won’t know how long immunity lasts until we have more data on how well the vaccine works.

The Pfizer and Moderna vaccines are very effective at preventing serious illness from the virus, however, it is 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 avoid close contact with others. 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.