The Covid-19 surge caused by the delta variant this summer is starting to slow, but some health experts are concerned that vaccine hesitancy combined with a potentially severe flu season could lead to a difficult winter.

How much worse will the 'delta surge' get? Watch these 7 factors.

Best-case scenario

Last winter, virtually no one in the United States was vaccinated against Covid-19. But according to CDC, as of Oct. 7, 56.2% of the U.S. population has been fully vaccinated—including 67.6% of U.S. adults and 83.8% of those ages 65 and older. And some unvaccinated people should have natural immunity, as there have been 44 million cases of Covid-19 in the United States, Vox reports. In addition, FDA will soon consider whether children as young as five will be eligible for a Covid-19 vaccine.

Now is the time to move the needle on flu vaccinations

While daily vaccination rates aren't what they were in April, they've increased over recent months, from around 500,000 per day in mid-July to about 950,000 a day in early October.

"Vaccination coverage is creeping up and that, combined with natural immunity, affords increasing protection," Jennifer Kates, director of global health policy at the Kaiser Family Foundation (KFF), said. "Kids should be eligible soon. Even with kids back in school, we are not seeing big outbreaks. These are all signs that we could be over the significant hump."

The Covid Scenario Modeling Hub projected what the next few months would look like if vaccine uptake among children is relatively strong and no new coronavirus variants emerge that are worse than delta. The model showed Covid-19 deaths continuing to fall from almost 2,000 per day in the last week of September to 90 per day in the last week of February 2022.

"It's no secret that Covid will likely remain endemic and still hurt people," said Kumi Smith, an epidemiologist at the University of Minnesota. "But if we can reliably send kids to school and give our health care systems a fighting chance, I think that will be a huge step."

Worst-case scenario

However, there is the chance that not many children get vaccinated. According to a KFF poll, 32% of parents in September said they plan to "wait and see" about getting their child vaccinated, while 7% said they'll get their child vaccinated only if required and 24% said they will "definitely not" get their child vaccinated.

Your top resources for Covid-19 readiness

If there is low vaccine uptake among children and a new coronavirus variant becomes dominant, the Covid Scenario Modeling Hub projected about 650 daily deaths by the end of February 2022 and trending up.

Even in states with high vaccination rates, there are still likely to be pockets of unvaccinated people who will keep transmission of the coronavirus alive, according to Josh Michaud, associate director of global health policy at KFF.

"Even in the most highly vaccinated U.S. states, there will still be a reservoir of people susceptible to the virus," Michaud said. "No state has reached a level of population immunity that could interrupt spread—and that may not even be possible, given the highly transmissible delta variant."

"Worst-case scenario is that another variant emerges," Kates said. "Covid has fooled us before and can again. And if people let their guard down—masking stopping too early, for example—we could see another surge, especially since we are entering winter."

On top of that, many health experts are concerned about a potentially severe flu season this winter. CDC Director Rochelle Walensky said that, since last year's flu season was historically mild, "population immunity is likely lower, putting us all at increased risk for disease this year especially among those most vulnerable, including our children."

In addition, a survey of more than 1,000 U.S. adults by the National Foundation for Infectious Diseases, (NFID) found that 44% of respondents are unsure about or are not planning to get a flu vaccine.

"We're particularly concerned because Covid is out there," William Schaffner, medical director for NFID, said. "Flu will come back this year. And we don't want to further stress our already very stressed health care system." (Scott, Vox, 10/8; Miller, USA Today, 10/7; Hoffman, New York Times, 10/7; Coleman, The Hill, 10/7; Fernandez, Axios, 10/7)