(CNN) — When Linda Stewart felt a tickle in her throat a few weeks ago, she got worried.

She's a 76-year-old woman, and she was well aware of the risks posed to her and her husband's health by Covid-19, flu and other colds that are sweeping the United States amid a rough respiratory virus season.

"I don't want to take any chances with my health," she said.

So far this winter, the rise in Covid-19 appears to be relatively mild -- hospitalizations are ticking up in most states, although the overall rate is still just a fraction of what it was during other surges.

But for older adults like Stewart, the situation is much more severe. Hospitalizations among seniors are nearing the peak from the Delta surge and rising fast.

And the age gap has never been wider. Since October, the Covid-19 hospitalization rate among seniors has been at least four times higher than average. Even during the first winter surge in 2020, when Covid-19 took a devastating sweep through nursing homes, there was never more than a three-fold difference.

Throughout the pandemic, a positive Covid-19 test for a senior has carried an extra heavy weight. Only about 13% of all reported cases in the US have been among people 65 and older, according to data from the US Centers for Disease Control and Prevention. But about half of all hospitalizations and three-quarters of all deaths have been in this age group.

The Covid-19 hospitalization rate for seniors has generally risen and fallen in line with broader trends, reaching a record high last winter during the Omicron surge and dropping significantly in the summer. But compared with other age groups, hospitalization rates have consistently been higher among the 65 and older population.

Dr. Eric Topol, a physician and professor of molecular medicine at Scripps Research, has dubbed the rise happening now the "senior wave."

"Right now we have an immunity wall built up against the Omicron family -- between shots and prior infections and combinations thereof -- that seems to be keeping younger folks in pretty good stead. But the immune systems of people of advanced age are not as strong," Topol said.

Younger adults who are immunocompromised are also likely experiencing disproportionately severe effects of the latest wave, he said, but there isn't sufficient data to understand trends in that population as well.

New variants that are more immune evasive and relatively low utilization of treatments like Paxlovid may have played a role in the rising hospitalization rate among seniors, Topol said.

But "the main culprit is booster deficiency" with rates that are "woefully inadequate," he said. "It all points to waning immunity. If more seniors had their booster, the effect would be minimal."

Vaccines help, and boosters still work

Stewart said that she's eased back on personal mitigation measures, but still keeps an eye on Covid-19 trends. She's found a balance between caution and contentment that she says works for her -- but getting her vaccines is really what helps her feel safest.

"I'm paying attention to the fact that it's picking up, so I'm a little bit more careful than I was, say, six weeks ago," she said. "With the pickup, I haven't reverted to how I was handling it a couple of years ago, but I'm more aware of who I'm around and maybe wearing my mask a little bit more than I used to."

A home test was negative for Covid-19 and confirmed by another test at a health care provider's drive through, which brought some relief, she said. But even if it was positive, knowing she was vaccinated and boosted gave her reassurance.

"That was the whole idea of being so proactive with all these vaccines. There was a very good chance that yeah, you might get sick, but you wouldn't get as sick as someone who didn't get all their shots and there was a really good chance you wouldn't end up in the hospital," she said. "So that really gave me a sense of security in some ways that even if I did get it, it wouldn't be really bad."

But most seniors are not as well protected as Stewart.

Just about a third of the 65 and older population has gotten an updated booster shot, according to CDC data -- a number that's concerningly low to public health experts.

"It's very, very concerning," said Dr. Preeti Malani, a physician at University of Michigan Health who specializes in infectious disease and geriatric medicine.

"There's a sizable number of people who actually got previous boosters who have not gotten this one and I worry that there's confusion, there's misinformation. So to seniors -- and to everyone -- I say: if you have not been boosted, go get boosted."

A recent survey from the Kaiser Family Foundation found that 60% of seniors were worried about a rise in Covid-19 cases and hospitalizations this winter -- a far larger share than average.

More than 40% were worried that they would get seriously sick themselves, but nearly as many said that they don't have plans to get the updated booster any time soon. In fact, nearly a quarter of seniors say they don't have any plans to get it at all, or will only get it if it's required.

A community-minded approach to protect the most vulnerable

Vaccines -- including the updated booster -- continue to prove to be effective at preventing severe disease. But booster uptake among seniors, while low, is much higher than it is for other age groups. Less than 10% of adults under 50 and less than 5% of children have gotten their updated booster, CDC data shows.

Still, experts say the gap in vaccination rates isn't enough to explain the large and growing gap in hospitalization rates.

"The truth is that, really, anyone can get this," Malani said. "But the older you are, the more likely you are to have severe symptoms, the more likely you are to be hospitalized, and the more likely you are to die."

Infectious diseases like Covid-19 don't spread differently among seniors than they do among younger people, experts say. Instead, family, friends and the broader community are often the ones bringing Covid-19 to seniors -- who are more likely to suffer more severe consequences.

"Seniors are the most at risk, but we bring it to them," Malani said. "A thing unique to older adults is that many of them are grandparents and many of them provide childcare for their grandchildren. So they sometimes get infected from their grandkids, who may also be going to school or daycare."

Many older adults live in congregate settings like nursing homes, which also present unique risks, she said.

But the point remains that seniors, while more vulnerable to severe outcomes, are not the main drivers of spread in the population.

A government watchdog report published earlier this month found that outbreaks in nursing homes were "strongly associated with community spread."

And nursing homes are particularly vulnerable again this winter. Weekly cases among residents have already surpassed all prior surges except the initial winter wave and the Omicron wave, and they continue to rise. But just 47% of residents and 22% of staff are "up to date" with vaccines, according to data from the US Centers for Medicare and Medicaid Services.

"We all would have hoped that we would have a vaccine that prevents transmission. We don't have a vaccine that does that, but it does reduce transmission and it does reduce severe outcomes," said Janet Hamilton, executive director of the Council of State and Territorial Epidemiologists.

For that reason alone, seniors who interact with other seniors should get vaccinated to help minimize severe outcomes, she said.

"But really, any individuals that come in contact with high-risk groups need to be the primary focus for getting vaccinated," she said.

The way forward isn't all or nothing

Stewart plans to host her family for Christmas again this year, for the first time since the pandemic started.

"We're careful about who we interact with. There isn't any undue risk that we felt in gathering with family. That's kind of our safe group," she said.

She and her husband also get together with small groups of friends that they trust are also vaccinated and similarly cautious, but they still plan to stay away from baseball games -- even though it's one of their favorite pastimes.

"We love going to baseball games. We're real fans, and we're very supportive of our team, but there's a lot of risk there. We take the ferry over and on that ride over, you're riding very closely with a lot of other people. And going to the ballpark, again, we're very close to a lot of unknown people," she said. "It's too risky still."

Malani, the infectious disease specialist, said she recently spoke with a friend who seemed to be seeking permission to gather with family this holiday season. She was eager celebrate in-person with loved ones after years spent about, but anxious about letting her guard down amid a rough respiratory virus season.

"It's about finding a balance, because the viruses are dangerous, but so is isolation," she said. "There's always a way forward and for now, it's through vaccination."

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