A single dose of the Pfizer COVID-19 vaccine sits on a table during a clinic.

After recommending in August that immunocompromised people get a third dose of the COVID-19 vaccine, the Centers for Disease Control and Prevention expanded its guidelines last week to include booster shots for other high-risk populations.

Here’s a rundown of who can get a booster shot, why they need it and where, locally, to get it:

Who can now get a booster shot of the COVID-19 vaccine?

On Friday, the CDC recommended people who are at higher risk of complications from COVID-19 get a booster shot, but only if their first two doses were by Pfizer-BioNTech.

That includes people who are 65 or older; residents of long-term care facilities; people with underlying medical conditions; and people between 18 and 64 who are at increased risk of COVID-19 exposure because of their job, such as health care professionals and other frontline workers.

What’s the difference between a booster shot and a third dose?

The COVID-19 booster is an additional dose of the vaccine given after the protection provided by the original shots begins to wane, and is designed to help people maintain their level of immunity for longer, according to John Hopkins Medicine.

A third dose of the COVID-19 vaccine, alternatively, helps protect people with weakened immune systems that did not develop a strong enough response to the first two doses.

On Aug. 12, the Food and Drug Administration authorized a third vaccine dose for those who are immunocompromised and received the vaccines by Pfizer or Moderna.

Those who qualify under the August amendment to the vaccines’ emergency use authorization include people receiving active cancer treatment for tumors or cancers of the blood; people with advanced or untreated HIV; and those who are taking immunosuppressant drugs, according to the CDC.

Why do people need a booster shot?

Dr. Aalok Khole, an infectious disease physician at Cheshire Medical Center in Keene, said recent studies have shown the vaccines’ overall effectiveness begins to wane after a few months, especially in the elderly and immunocompromised.

And because many of those now eligible to receive a booster shot were among the first to receive their initial doses, he said a booster shot will help strengthen their protection against COVID-19.

“Vaccines continue to remain effective in preventing severe disease, hospitalization and death,” Khole said in an email. “However, [this] approach is evidenced-based and on the lines of being proactive rather than reactive.”

What if I fall under one of those populations needing a booster, but didn’t get the Pfizer vaccine?

It’s likely you will need a booster shot, but it’s not available at this time, according to the CDC. In coming weeks, the federal health agency says it will continue evaluating the available data to determine next steps for people who received the Johnson & Johnson or Moderna vaccines.

Khole stressed the importance of these people continuing to wear a mask in public, practicing proper hand washing, physical distancing when possible and staying home when sick.

When should I get the booster shot?

The N.H. Department of Health and Human Services hopes to have booster shots provided to health care providers and pharmacies this week, according to spokeswoman Laura Montenegro.

Third doses have been available since Aug. 12.

The CDC recommends the booster shot be administered at least six months after receiving a second dose. People needing a third dose should do so at least 28 days after the second dose.

Where can people get a booster shot?

There are more than 500 locations across New Hampshire where people can get a third dose, including pharmacies and hospitals, according to Montenegro. These facilities will also offer the booster shot once it’s distributed.

Locally, Cheshire Medical patients can call their primary-care or specialty provider’s office — if they meet the guidelines to request a third dose — and will receive a call back for an appointment, spokeswoman Heather Atwell said.

Monadnock Community Hospital in Peterborough did not respond to a request for comment Monday on plans for booster shots or third doses.

As with the initial shots, appointments for a third can also be made at vaccines.nh.gov, by calling the state’s hotline at 2-1-1 or by directly contacting your primary-care provider or local pharmacy.

Before making an appointment, people should consult with their health care provider to determine if the booster shot and third dose is right for them, according to the CDC.

What will a COVID-19 booster shot cost me?

Like the initial doses of the vaccine, booster shots will be free, regardless of insurance status, President Joe Biden announced on Friday.

When can other people get a booster shot? Will everyone need one?

Khole said it’s hard to say at this point whether everyone will need a booster shot or not. Several factors will influence this, including emerging data on vaccine performance in the general public, any new variants and considerations of vaccine availability and equity.

The CDC says it’ll continue evaluating how well the vaccines are working for different populations.

Questions about the COVID-19 vaccines or anything else related to the viral disease can be answered at vaccines.nh.gov or by calling the state’s hotline at 2-1-1.

Olivia Belanger can be reached at 352-1234, extension 1439, or obelanger@keenesentinel.com. Follow her on Twitter @OBelangerKS.