A government shutdown would sideline important cancer and infectious disease research at the CDC amid the pandemic, former CDC director Tom Frieden told ABC News.

Frieden led the CDC during the Obama administration and through the 16-day government shutdown in 2013.

"Although people who are directly responding to a pandemic would be exempt from furlough, it harms the rest of the agency and makes it harder to do everything ... whether it's answering emails, responding to requests, helping to investigate problems. It has a dampening effect on the agency being to able to do its job protecting Americans," he said.

"The staff at the CDC have basically been fighting the pandemic for 18 months. This would add insult to injury," Frieden said. "We had to furlough people who were working on prevention of cervical and breast cancer ... on the prevention of heart attacks and strokes ... on people collecting data on the health of Americans. And all of that makes people less safe and corrodes agency morale."

-ABC News' Ben Siegel

The CDC on Wednesday issued an urgent health advisory to try to increase vaccinations among women who are pregnant, recently pregnant or may become pregnant.

Just 31% of pregnant people have been vaccinated, the CDC said.

As of Monday, pregnant people accounted for 125,000 COVID-19 cases and 161 deaths, the CDC said.

"Cases of COVID-19 in symptomatic, pregnant people have a two-fold risk of admission into intensive care and a 70 percent increased risk of death," the CDC said.

The agency said it's calling "on health departments and clinicians to educate pregnant people on the benefits of vaccination and the safety of recommended vaccines."

Approximately 118 million Americans remain completely unvaccinated, including 70 million over the age of 12, according to federal data.

On Wednesday, the U.S. is likely to reach a significant vaccine milestone of 200 million adults vaccinated with at least one dose.

Alaska currently has the country's highest infection rate with daily cases up by nearly 2,500% since mid-July, according to federal data.

West Virginia, once a model for its vaccine rollout, now has more hospitalized patients than at any point in the pandemic.

But states including Florida, Mississippi, Louisiana and Georgia appear to be improving, federal data show.

Nationwide, around 79,000 people are hospitalized, down by more than 20% since the beginning of September. Even so, ICU capacities in several states, including Alabama, Georgia, Idaho, Kentucky and Texas, remain at critical levels.

-ABC News' Arielle Mitropoulos

United Airlines plans to immediately terminate up to 593 employees who chose not to get vaccinated under the airline’s mandate.

United said more than 99% of its employees chose to get vaccinated.

Roughly 2,000 employees sought a medical or religious exemption. Because of ongoing litigation, United said it will not reveal how many employees were granted exemptions. United says anyone who applied for an exemption will still be allowed to go to work until a final decision is made on how to handle their cases. The airline expects a decision by Oct. 15.

-ABC News' Sam Sweeney