Why there are so many COVID-19 cases, hospitalizations in the Northeast

As COVID-19 cases and hospitalizations rise across the United States, the Northeast is being hit much harder than other regions of the country.

States including Connecticut, Massachusetts, New Jersey, New York and Rhode Island have some of the highest weekly case rates per 100,000 in the nation, according to data from the Centers for Disease Control and Prevention.

What's more, while seniors across the U.S. are being hospitalized at a rate three times higher than other age groups, it's five times higher in the Northeast, CDC data shows.

Public health experts said the reason for the rise is multi-factorial and includes the spread of more transmissible variants, a higher proportion of elderly people and not many Americans receiving the updated bivalent booster.

Spread of XBB.1.5

Not much is known about XBB.1.5. What we do know is that it is an offshoot of the omicron subvariant, which caused a surge of infections during the winter 2021-2022 season.

Early research suggests it may be better at evading immune responses and, recently, the World Health Organization said it was the most transmissible COVID variant to date.

"I think that the primary thing right now that's driving higher rates, perhaps, in the Northeast is the higher proportion of XBB.1.5," Dr. Shira Doron, chief infection control officer for Tufts Medicine Health System, told ABC News. "That's the number one detectable difference between the Northeast and the rest of the country."

CDC data shows that in the U.S. Department of Health and Human Services' Region 1 -- including Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont -- XBB.1.5 accounts for an estimated 71.6% of new COVID infections.

In Region 2 -- made up of New York, New Jersey and the U.S. territories in the Caribbean -- it's an estimated 72.7%.

Comparatively, XBB.1.5 makes up an estimated 7.3% of new cases in the Midwest and about an estimated 6% of new cases in the Great Plains region.

Low booster rates

As of Jan. 5, 2022, six of the 10 states with the highest percentage of residents who have received an updated bivalent booster dose live in the Northeast, according to CDC data.

Vermont leads the pack with 31.3% followed by Maine, Massachusetts, Connecticut, Rhode Island and New Hampshire, respectively.

The shot was developed to specifically target the BA.4 and BA.5 variants. It is currently unclear how well the booster protects against XBB.1.5.

"This XBB.1.5, it is an omicron offshoot so there is hopefully some cross reactivity in our antibody and immune responses in those who got the BA.4/BA.5 bivalent booster," Dr. Scott Roberts, an assistant professor and the associate medical director of infection prevention at Yale School of Medicine, told ABC News. "It probably won't be as good as BA.4/BA.5, but it's certainly the next best thing."

Even though the rates are higher in the Northeast than elsewhere in the country, they are still lower than experts would like to see and having less than half the population vaccinated is only so helpful.

"My suspicion is that the transmissibility of XBB.1.5 most likely far outweighs the proportion of people who have gotten the booster," Roberts said. "Even though we are a higher vaccinated region than many other regions of the country, it is still the minority of people in our region who have gotten the bivalent booster."

Higher proportion of older adults

Another reason for the increase in cases and hospitalization could be that the Northeast has a higher proportion of elderly people who are more prone to infection, severe disease and death from COVID-19, experts suggest.

Although the majority of seniors aged 65 and older are fully vaccinated at 94.1%, just 38.1% have received an updated bivalent booster dose, according to CDC data.

Age is a known risk factor and the immune system wanes as a person ages, making them more vulnerable to infection.

"What I think is fascinating is if you look at the top 15 states in each of the 50 states, five of them are in New England," Dr. Leonard Mermel, an infectious diseases expert at Rhode Island Hospital, told ABC News. "Five of the top 15 states with the greatest proportion of the population 65 years of age or older are in New England: Maine, Vermont, New Hampshire, Rhode Island, Connecticut."

"And so, for older folks, you take their waning function their immune system, and not enough Americans getting boosted, and that's problematic," he added.