The number of COVID-19 infections in Maine has jumped 35 percent since July 1 as the delta variant has swept across every corner of Maine, leading to record numbers of hospitalizations and a rising death toll.

While Maine has among the highest vaccination rates in the nation, some fully vaccinated people are still contracting the coronavirus – and some have been hospitalized or died.

This Q&A takes a look at the breakthrough cases, what is behind the numbers and how booster shots fit into this latest chapter of the pandemic.

What is a breakthrough case? And why does it happen?

A breakthrough case is when a fully vaccinated person – someone who has had a final dose for at least two weeks – tests positive for COVID-19. As of Oct. 1, there had been at least 4,167 breakthrough infections in the state, according to the latest data from the Maine Center for Disease Control and Prevention.

As for why they happen, breakthrough cases are both expected and inevitable in the midst of a still-developing pandemic caused by a still-evolving virus. That’s because the vaccines do not prevent infection with the coronavirus. Instead, vaccines are designed to teach the body how to fight off an infection by triggering an aggressive immune system response that will hopefully prevent severe illness.

Put another way, vaccines are akin to an army training for an invasion: the virus may still make landfall, but vaccines are the training that helps keep the invaders from getting beyond the beach.

Why is the number of breakthrough cases rising in Maine?

It’s a matter of numbers, according to health officials. Roughly two-thirds of Maine’s 1.3 million residents are fully vaccinated, which is one of the highest rates in the nation. Those 4,167 breakthrough cases represent just 0.5 percent of the more than 880,833 fully vaccinated Mainers, based on the latest Maine CDC data.

The 4,167 cases also represent 7.5 percent of the 55,489 new infections reported in Maine since the first date in late-January when people in Maine could be fully vaccinated. In other words, the vast majority of new COVID cases are coming from the minority who are not vaccinated.

Dr. Nirav Shah, director of the Maine CDC, seized on the fact that more than 99.5 percent of fully vaccinated Mainers have not reported breakthrough cases.

“So I just want to note that although they happen, they are generally uncommon as compared to the substrate level of people who are fully vaccinated in the state,” Shah said Wednesday. “And that’s a function of just how remarkably good the vaccines are.”

Do breakthrough cases show that vaccines aren’t effective?

No, and hospitalization figures help illustrate this point.

Again, the vaccines help prevent severe illness – and death – but do not block infection. Depending on the week, anywhere from 65 to 75 percent of people in the hospital because of COVID-19 have been unvaccinated, even though just one-third of the population is unvaccinated.

Further crunching those numbers, the roughly 160,000 children in Maine who are too young for vaccination thankfully account for very few of the hospitalizations. So two-thirds to three-quarters of hospitalizations in Maine in recent weeks are among the roughly 22 percent of Mainers (about 300,000 people) who are eligible but not yet vaccinated.

Vaccine effectiveness does decrease over time, however, which is why most vaccines require periodic booster shots. That may be every decade for a tetanus shot, while physicians recommend annual flu shots because new strains of the influenza virus emerge. And, as we’ve seen with the highly contagious delta variant, this coronavirus can mutate quickly.

Recent medical studies have suggested that some of the COVID vaccines could become less effective over time.

A study published by the U.S. CDC in mid-September found that the Pfizer vaccine was 77 percent effective at preventing hospitalizations after four months among people without compromised immune systems compared to 91 percent effective initially. But another study, published a month earlier, found no loss of effectiveness at preventing severe illness six months after a final dose of Pfizer or Moderna.

“Protection against severe COVID-19 resulting in hospitalization was sustained through 24 weeks after vaccination with mRNA COVID-19 vaccines,” the authors wrote in the study published by the U.S. CDC in August. “To reduce their risk for hospitalization, all eligible persons should be offered COVID-19 vaccination.”

Asked Wednesday about waning effectiveness, Shah summarized the research: “There has been a little bit of waning immunity for infections, but, thankfully, all three vaccines continue to show strong effectiveness as it relates to serious outcomes like hospitalizations and ventilators. What that means in effect, according to the U.S. CDC, is that what’s driving many of the breakthrough hospitalizations is more the fact that there are more vaccinated people and less the fact that the vaccine is losing its effectiveness.”

But breakthrough hospitalizations do occur and their numbers are rising. Why?

Officials say that’s likely the result of Maine’s large number of fully vaccinated people combined with complicating health factors in specific individuals.

As of early Wednesday, 18 of 51, or 35 percent, of the COVID hospital patients within the Northern Light Health network were fully vaccinated. Vaccinated individuals accounted for eight of the 20 critical care patients within Northern Light’s network, which includes Eastern Maine Medical Center in Bangor and Mercy Hospital in Portland.

Dr. James Jarvis, COVID-19 incident command physician for Northern Light, said while it is not true for every single person, most of the fully vaccinated people in the hospital or ICU had underlying medical conditions. The same is not true of many unvaccinated patients.

“What I can say is when we look at our unvaccinated population, they are becoming a younger, healthier crowd of individuals who are requiring hospitalization,” Jarvis said. “And that has been the case for the last few months.”

How does Maine’s rate of breakthrough infections compare to other states?

That’s difficult to gauge because states track and report breakthrough cases differently – and some don’t report them publicly at all, unless they are requested.

Neighboring New Hampshire reported 1,809 COVID infections among fully vaccinated people as of Sept. 15, roughly 3.3 percent of the total cases since mid-January, according to the TV station WMUR. Massachusetts, meanwhile, reported this week a total of roughly 40,500 breakthrough infections, including 3,741 since last week, according to a report in The Boston Globe. That translates to less than 1 percent of Massachusetts’ fully vaccinated population, the newspaper reported.

But the U.S. CDC does not require states to report breakthrough infections, and there is no federal standard for tracking them. The federal agency, for instance, regularly reports only the total number of hospitalizations and deaths among fully vaccinated people. Nationwide, those figures were 16,889 hospitalizations and 5,226 deaths as of Sept. 27, according to the most recent data.

The incomplete and inconsistent data is frustrating for researchers and groups tracking COVID-19.

“States are significantly hampering efforts to define the national landscape of COVID-19 breakthrough cases by not reporting the data, or reporting it infrequently in atypical formats,” wrote Beth Blauer, an associate vice provost at Johns Hopkins University who has helped lead that institution’s closely watched COVID tracking program. “The delta variant is already ravaging the United States, and other variants may arise in the future. We need detailed, real-time information on breakthrough cases to monitor for vaccine efficacy and to defend against new surges.”

How do booster shots fit into this discussion about breakthrough cases?

The U.S. CDC has recommended booster shots for people 65 or older who received the Pfizer vaccine, and for younger people with underlying health conditions that could put them at higher risk of serious illness or death from COVID. Federal officials also have authorized booster shots for people with higher-exposure jobs or living situations, such as health care workers and teachers or day care providers, as well as grocery store workers and people living in homeless shelters.

Federal regulators also are evaluating the need for Moderna or J&J boosters.

Booster shots are now widely available for eligible populations in Maine. In addition to some doctors’ offices, retail pharmacies like CVS and Walgreens, as well as most major health care networks – including MaineHealth and Northern Light Health – are offering free booster shots.

As of Wednesday, 35,569 booster shots or additional doses of vaccine had been administered in Maine.

The state is averaging about 2,700 booster shots a day this week, compared to around 1,000 daily last week. Shah said those numbers, along with the number of first and second doses being administered, show that vaccines are still widely and easily available throughout Maine.

The state’s primary focus remains on getting those first and second shots into arms, which Shah said will provide the “biggest bang” from a population-wide public health standpoint because unvaccinated people are more likely to contract and spread the virus.

“Getting the booster is important. You should walk and not run there,” Shah said. “The thing that drove the decision around boosters was that they were starting to see more breakthrough cases, but not necessarily more breakthrough hospitalizations after adjusting for population and time since vaccination. But in a sense, the rationale behind boosters was, in part, to prevent the vaccine from losing that effectiveness.”

Information on vaccination sites is available at maine.gov/covid19/vaccines/vaccination-sites or by calling the state’s vaccination hotline at 888-445-4111.