In the United States, more than 72 million people, or 21% of the population, have received at least one dose of the Covid vaccine, according to data from the Centers for Disease Control. And President Joe Biden recently said he will direct states to make all adults, ages 18 and up, eligible for the coronavirus vaccines by May 1.

There are currently three coronavirus vaccines authorized by the Food and Drug Administration for emergency use, from drug makers Pfizer-BioNTech, Moderna and Johnson & Johnson.

As vaccine supply increases and more people become eligible for the shot (to check your eligibility status, you can use NBC News' plan your vaccine tool), you might have questions about the vaccination process beyond the logistics of how to get an appointment. Here's what you need to know.

Table of Contents

• How vaccines for Covid work
• How to get a vaccine appointment
• What to expect during your vaccine appointment
• Possible side effects
• When immunity kicks in and how long it lasts
• What you can do once fully vaccinated
• When will America reach herd immunity

How vaccines for Covid work

The Covid vaccines from Moderna and Pfizer use an innovative vaccine technology called messenger RNA, or mRNA, which gives cells instructions on how to make a non-infectious piece of the coronavirus' spike protein. Once the immune system detects the copies of the spike protein, it creates antibodies against it. And in the future, if you're exposed to the coronavirus, your body can recall how to make the antibodies to fight it.

In clinical trials, Moderna's vaccine was 94.1% effective at preventing laboratory-confirmed Covid-19 illness in people who received both doses, and the Pfizer-BioNTech vaccine was 95% effective.

J&J's Covid vaccine, on the other hand, uses adenoviruses, a type of virus that causes the common cold, as a vehicle to deliver instructions to cells about how to fight the virus. Similarly, when the vaccine is injected into people's arms, it triggers the immune system to create antibodies.

J&J's vaccine was 72% effective against moderate to severe Covid-19 infection in the U.S., and 66% protective against moderate and severe disease overall worldwide.

How to get a vaccine appointment

Each state has its own plan for vaccine eligibility, so it's a good idea to start with your state or local health department's website. You can also call your doctor if you're not sure if your specific conditions or underlying factors meet the criteria.

The CDC's VaccineFinder tool can point you in the direction of providers and pharmacies near you that have the vaccines. (Private practices are not currently receiving vaccines for patients, but your doctor may have more insight about how to go about getting an appointment.) You need an appointment in most locations to get your vaccine.

There may be additional volunteer-run resources aimed at getting a vaccine appointment available to you depending upon where you live. In New York City, for example, the website TurboVax helps match people with government-run vaccine sites in the area. Similarly, VaccinateCA helps California residents.

President Biden announced on March 11 that a federally-supported find-a-vaccine website would be available by May 1. "No more searching day and night for an appointment for you and your loved ones," he said during the prime-time address.

What to expect during your appointment

There has been a lot of excitement and anticipation surrounding vaccine appointments. If you're someone who is afraid of needles, focus on taking deep breaths to relax in the moment and consider chatting up the person administering the shot to provide a distraction.

After your shot, you will be asked to wait for 15 minutes to monitor for reactions. You'll also receive a card that tells you which vaccine you got, and the date when you're due back for your second dose.

Possible side effects

It's normal to experience some side effects from the Covid vaccines a few days after the fact; that's a sign that your immune system is working. Young people tend to have more intense side effects than older folks, because young people's immune systems are more robust.

Common side effects include pain near where the vaccine was injected, redness and soreness, as well as fatigue, headache, chills, fever and nausea, which can last up to a few days.

For both the Pfizer and Moderna vaccines, side effects tend to be more frequent after getting the second dose, which makes sense: the first dose is meant to trigger an immune response, and the second dose builds upon it.

The reported rate of fever and chills was more than four times higher after the second dose of Pfizer's vaccine compared to the first, according to data from the CDC's Vaccine Adverse Event Reporting System collected between Dec. 14 and Jan. 13.

In Moderna's clinical trials, less than 1% of people reported a fever after the first dose, but 15.6% of vaccine recipients had a fever after the second.

The CDC says that it's okay to take OTC pain medications after getting the vaccine to cope with some of the side effects, but it's not recommended to take it beforehand because it could dampen the vaccine's effectiveness.

You may have heard also about European countries stopping the use of the Oxford-AstraZeneca Covid-19 vaccine (which is currently not in use in the U.S.) after reports that a small number of people developed a particular blood clot after receiving at least one dose.

The World Health Organization said Monday that "there is no evidence that the incidents are caused by the vaccine." Experts say that the number of blood clot cases detected is "much lower than would be expected to occur naturally in a general population of this size and is similar across other licensed COVID-19 vaccines," in a statement Sunday.

The European Medicines Agency approved the AstraZeneca vaccine Thursday, noting that the benefits of protecting people from Covid outweigh the possible risks.

When immunity kicks in and how long it lasts

It takes your body some time to build an immune response after getting a vaccine, about two to three weeks, Dr. Andrew Badley, the Mayo Clinic's Covid Research Task Force Chair said on Feb. 17.

If you're getting one of the two-dose Covid vaccines, that means you'll have partial immunity to Covid about two weeks after the first dose.

Then, 10 to 14 days after the second dose of both two-dose vaccines, "you get a 10-fold increase in neutralizing antibodies," Dr. Anthony Fauci, the nation's leading expert on infectious disease, daid during the Precision Medicine World Conference on Feb. 12.

In clinical trials, J&J's vaccine showed protection against Covid-related hospitalization and death starting 28 days after vaccination. In J&J's clinical data, neutralizing antibodies for Covid were picked up in 90% of the people after 29 days, and 100% of people 56 days after receiving the shot.

So a person is considered "fully vaccinated" for Covid two weeks after receiving the second dose of a two-dose vaccine (such as Moderna and Pfizer) or two weeks after getting the single-dose J&J vaccine, according to the CDC. Any time before that point, you aren't fully protected and need to continue practicing social distancing and wearing a mask.

At this stage, it's unclear how long vaccine-induced immunity from Covid lasts. But Fauci said that antibodies can last for at least six months and potentially a few years. There are also other forms of immunity that kick in, like T-cells, that may help with more extended immunity, but that is currently being studied.

What you can do once fully vaccinated

There's emerging evidence that fully vaccinated people are less likely to transmit the virus to others, according to the CDC. Some early data from Israel that suggests that the Pfizer vaccine reduces transmission. And in J&J's trials, they found a 74% reduction in developing asymptomatic infection, which indicates that the vaccine reduces transmission, former FDA commissioner Dr. Scott Gottlieb told CNBC's "Squawk Box" on March 1.

The CDC recently released new guidelines for people who are fully vaccinated, saying that it's safe for fully inoculated to visit with other people who are also fully vaccinated as well as some unvaccinated people indoors without wearing masks or social distancing, according to the guidelines.

But given that only 11% of the U.S. population is fully vaccinated, and there's not enough data on the effect of vaccines on transmission, it's important to keep up with safety measures outside of the home even if you got your vaccine.

"Everyone — even those who are vaccinated — should continue with all mitigation strategies when in public settings," CDC Director Dr. Rochelle Walensky said in a statement March 9.

When the U.S. could reach herd immunity

Herd immunity is used to describe the point at which enough of a population is immune to an infectious disease through vaccination or infection that the virus has difficulty spreading.

Adam MacNeil, an epidemiologist at the CDC said in late February that the U.S. was "nowhere close" to herd immunity. Experts like Fauci have said that the threshold of immunity required to provide an "umbrella" of protection is between 75% and 85%.

There is expected to be enough vaccine available for all adults beginning in May. And getting kids vaccinated could be an important step in reaching herd immunity.

Moderna and Pfizer are testing their vaccines on kids. Fauci said that there will likely be enough data to start vaccinating children under 12 by the first quarter of 2022, during a hearing with the House Committee on Energy and Commerce Wednesday. High schoolers may be able to get vaccinated by the fall.

But, according to Fauci, "we should not get so fixated on this elusive number of herd immunity," he said during a press briefing Monday. "We should just be concerned about getting as many people vaccinated as quickly as we possibly can because herd immunity is still somewhat of an elusive number."

This story has been updated to include that the EMA approved AstraZeneca's vaccine Thursday.

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