While vaccination against infectious diseases such as measles initially took a dive during the onset of COVID-19, vaccines took center stage again when they helped to turn the tide of the pandemic.

Immunization is critical to public health, but to combat outbreaks, there needs to be sufficient immunity both individually and across the wider population. Booster doses often fill in those gaps.

As health officials work to bolster steadily rising immunization rates to pre-pandemic levels and to vaccinate as many people as possible against COVID-19, vaccine boosters could play a role in keeping everyone safe.

An initial vaccine educates your body's immune system about what's in it, building immunity just as a natural infection would but without the disease, Cedars-Sinai immunologist Dr. Suzanne Cassel explains. Booster shots strengthen or maintain those immune responses.

Most traditional vaccines expose your body to the virus they're targeted against—either a small amount or part of the live virus or a weakened or dead version. The COVID-19 vaccines instead focus on a specific protein that is only found on the surface of SARS-CoV-2, the virus that causes COVID-19.

The single-dose Johnson & Johnson (Janssen) vaccine against COVID-19 is a viral vector vaccine, which uses a different, safe virus to produce a spike protein that enables the virus to enter your cells. On the other hand, the two-shot Pfizer-BioNTech and Moderna vaccines use messenger RNA (or mRNA) to teach your cells how to make the spike protein and then delete the instructions. Your cells then create their own spike proteins.

After a virus or proteins are introduced, they kickstart a response in your white blood cells (known as T and B cells) that detects the invader and produces antibodies against it. Those cells then remember how to respond if they come across it again, Dr. Cassel says.

Some B cells go on to become long-lived plasma cells, which constantly produce large quantities of antibodies, while others become memory B cells that lie dormant until exposed to infection. Repeated exposure turns those memory cells into plasma cells, increasing antibody production. It also causes mutations in how antibodies are made that can make them more effective.

"By interacting with the different cells, the new antibodies that are stronger and bind better are selected," Dr. Cassel says. "We get more bang for our buck. We get better antibodies, and we get more antibodies."

The flu vaccine is another type of booster. Scientists develop a modified version every year to respond to the strains of influenza most expected to circulate that season.

Read: Staying on Top of Your Child's Vaccines During COVID-19

Most vaccines will offer immunity for years, although lifespans vary, Dr. Cassel says.

Scientists have carefully studied how long each individual vaccine dose will protect you and have developed vaccine schedules with that in mind. Dr. Cassel adds that if you get a follow-up vaccine too soon, you can develop a painful local reaction. But if you wait too long for certain vaccine doses, infections start to rise.

Inactivated vaccines (containing dead virus) typically offer less protection than live vaccines, meaning you might need more over time to build your immunity, according to health officials. For example, the Centers for Disease Control and Prevention (CDC) recommends all adults get a booster dose of the tetanus, diphtheria, pertussis vaccine (Tdap) every 10 years.

Read: Do I Really Need a Flu Shot?

People with certain risk factors for infectious disease—such as those living in outbreak settings or seniors—benefit from booster doses. Because your immune system usually wanes as you age, the CDC recommends all adults over the age of 65 get vaccinated against pneumococcal disease (such as pneumonia), even if already vaccinated.

And the CDC suggests shingles vaccines for healthy adults age 50 and older, regardless of previous chickenpox infection or vaccination. Repeated exposure to the varicella-zoster virus, which causes both chickenpox and shingles, would normally continue to bolster defenses against the virus in your body, Dr. Cassel explains. But because chickenpox vaccination is now so widespread, adults rarely encounter the virus in the wild.

"Boosters take the place of natural exposure," Dr. Cassel explains. "We've done such a great job of eradicating these horrible, deadly diseases that we don't see the natural infection anymore or we see it very infrequently.

"So, we can't trust that we're just going to boost ourselves," she says.

Read: The Pneumonia Vaccine Explained

It's still unclear.

As of July 8, 2021, the CDC and Food and Drug Administration say fully vaccinated Americans don't need any further doses after the one- or two-shot series,  but are monitoring the situation—and "are prepared for booster doses if and when the science demonstrates that they are needed."

"We know at this point that COVID-19 vaccines are protective for at least six months," Dr. Cassel says. "Why do we know it's only six months? Because that's all we've got so far.

"We have no idea how long these vaccine memory cells are going to last and how many we need to be sufficient to squelch a very aggressive virus," she adds.

Health officials are closely following the data, including whether existing vaccines will keep people safe from new variants as COVID-19 continues to evolve.

Read: Vaccines Save Lives

"The better and more rapid our antibody response is, the more quickly the virus is going to be stifled," Dr. Cassel stresses.

That means fewer, less severe or no symptoms if you're exposed—and reduced transmission. This helps to protect vulnerable people who aren't able to get vaccinated, such as people who are on immunosuppressants or have had transplants.

"If we get herd immunity, then there's so little virus walking around that it just can't get a foothold in our community," she says.