As public health experts continue to debate who should receive booster shots of Covid-19 vaccines, some parts of the country are seeing surges of cases, driven in part by breakthrough infections. Concerned about waning immunity, three states are expanding booster shot eligibility to all adults who want one.

Radio Advisory episode: The unanswered vaccine questions you should be tracking

How common are breakthrough infections? It's hard to tell.

Until recently, there was little nationwide data on breakthrough infections due to CDC's decision to only track breakthrough infections resulting in hospitalization or death.

However, last month CDC started publishing Covid-19 case and death rates by vaccination status for the first time, though the data was only current as of Sept. 4, and the agency's data on breakthrough hospitalizations by vaccination status only measured through August, MedPage Today reports.

"It's definitely a problem that here in the U.S., we have not had real-time, reliable data on breakthrough infections," said Leana Wen, a professor of health policy and management at George Washington University. "Why should we be waiting until we see Americans end up in the hospital with breakthrough infections before we take action and recommend boosters more broadly?"

In a statement, CDC said it would be updating its breakthrough case and death data in mid-November to be current through Oct. 2. CDC explained the lag by saying, "Deaths usually occur up to 30 days after diagnosis. This is why CDC allows at least four weeks of lag time to link case surveillance data to Immunization Information System (IIS) and vital records data up to a month after the diagnosis," MedPage Today reports.

What state data can tell us

Some states are keeping regular data on breakthrough infections, including Vermont, which has seen an uptick in Covid-19 cases recently.

Almost 72% of adults in Vermont are fully vaccinated—the highest vaccination rate in the country. However, the state also had the 12th-highest rate of new Covid-19 cases over the past week, according to state data, with the seven-day average of new cases rising 42%.

According to Mark Levine, Vermont's health commissioner, there's not "one simple answer" behind the virus' surge, though experts say the delta variant is a major contributor. Levine added that residents who were "efficiently and effectively" vaccinated early are likely seeing their immunity wane, which he also believes is contributing to the rise in cases.

The unvaccinated people in Vermont are also likely a major cause of the rise in cases, according to John Brownstein, an epidemiologist at Boston Children's Hospital.

"You still have pockets of unvaccinated people, even in a highly vaccinated state," he said. "Unvaccinated individuals are the primary host by which the virus will spread and continue to allow for transmission to take place in the community and ultimately create challenges for those that are vaccinated."

Colorado also keeps tabs on breakthrough infections, updating their data daily, MedPage Today reports. As of Nov. 9, 299 vaccinated people and 1,127 unvaccinated or not fully vaccinated people were in the hospital with Covid-19—meaning about 21% of the state's Covid-19 hospitalizations were the result of breakthrough infections.

In response to the coronavirus' spread throughout the state, Colorado Gov. Jared Polis (D) on Thursday said all adults are eligible for a booster shot, provided enough time has passed since their last dose.

"We want to ensure that Coloradans have every tool they need to protect themselves from this deadly virus and to help reduce the stress on our hospitals and health care workers," Polis said.

Similarly, the California Department of Public Health announced that all adults who wish to get a booster shot can.

"Do not turn a patient away who is requesting a booster," Tomás Aragón, director of the department of health, said in a letter. "Allow patients to self-determine their risk of exposure."

New Mexico has also extended Covid-19 booster eligibility to all adults. Gov. Michelle Lujan Grisham (D) on Friday said she "strongly encourage[s] every New Mexican to register for a booster today—we have appointments available and are ready to get shots in arms."

David Scrase, acting secretary of the New Mexico Department of Health, described New Mexico as "a high-risk setting."

"Case counts are significant, spread rates are far too high, and the delta variant is far more transmissible than previous variants," Scrase said. "In addition, our hospitals are well beyond capacity, and several have declared Crisis Standards of Care."

Are booster shots necessary for everyone?

Experts are divided on whether all adults should be eligible for booster shots.

Initially, many thought that booster shots would be necessary only for certain vulnerable segments of the population, but that rationale has changed, said Colleen Kelley, an associate professor of medicine in the division of infectious diseases at Emory University School of Medicine.

The coronavirus continues to spread throughout the country, largely driven by the millions of Americans who aren't vaccinated, and vaccine protection is waning more than experts thought it might, Kelley said. That means breakthrough infections are more likely, and booster shots can increase antibody levels to help prevent those infections.

The Biden administration is pushing to offer booster shots to everyone, the Washington Post reports. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said he supports booster shots for all "given that we are starting to see plateauing and even in certain states, an actual increase in cases."

"I have always been and still am in favor of boosters," Fauci added. "Exactly who gets the boosters is up to the FDA and up to the CDC." Pfizer and BioNTech last week said they requested FDA to expand its authorization for booster doses of the companies' Covid-19 vaccine to include anyone age 18 and older.

Other experts have been more hesitant in the booster debate. Eli Rosenberg, deputy director for science in the Office of Public Health at the New York State Department of Health, said he worries focusing on boosters will distract from the most important goal—getting the unvaccinated vaccinated.

"It's easy with all the discussion about boosters to lose that really important message that the vaccines are still working," Rosenberg said. He added that getting vaccinated "is still the critical step."

David Dowdy, an epidemiologist and associate professor at the Johns Hopkins Bloomberg School of Public Health, noted that it's important to remember the vaccines still are very effective at preventing severe Covid-19 and death. He compared getting Covid-19 after getting vaccinated to getting the flu, with a few exceptions.

"We have been willing to have holiday celebrations in the midst of flu season from the very beginning of time, for our country at least," Dowdy said. "But at the same time, we are dealing with a new disease that we don't have all the long-term data [for], and we don't know if we're going to be seeing a new wave." (Putka, MedPage Today, 11/11; Deliso, ABC News, 11/12; Hassan et al., New York Times, 11/12; McGinley et al., Washington Post, 11/11; Stieg, CNBC, 11/10; Walker/Holder, New York Times, 11/11; Franklin, NPR, 11/11; Lonas, The Hill, 11/13)