Not enough Americans are being vaccinated against Covid, the flu and RSV to stem rising numbers of the respiratory illnesses, the Centers for Disease Control and Prevention said Thursday.

The agency issued a health alert to doctors across the country, warning that low vaccination rates amid “ongoing increases in national and international respiratory disease activity” could strain U.S. health care systems in the coming weeks, and called on doctors to encourage their patients to get the shots immediately to protect them for the remainder of the season.

“In the past 4 weeks, hospitalizations among all age groups increased by 200% for influenza, 51% for Covid-19, and 60% for RSV,” the CDC said in its health alert. “Currently, the highest respiratory disease activity in the United States is occurring across the southern half of the country, with increasing activity in northern states.”

As of Dec. 8, just 17.2% of adults had received the updated Covid shot, according to the CDC. About 40% of children and adults have gotten this year’s flu shot. And 15.9% of older adults eligible for the RSV vaccine have received it.

Rising illnesses in children

The warning comes as illnesses are increasing among children — including an apparent uptick in a rare Covid-related complication called MIS-C, the CDC said. Cases had appeared to drop in recent years.

But from September to mid-December, the CDC received 30 such reports in kids, reflecting a “relative increase” compared with previous months.

RSV is also surging as a key drug in helping to keep babies safe from the virus has been in short supply.

Doctors should not hoard the few doses they have of the drug, called Beyfortus, the CDC said, and instead give all they can now, rather than wait for the situation to possibly worsen later this season. The CDC recommends Beyfortus for babies and toddlers up to 19 months old.

Read Related Also: Raging bulls! Trump slams 'total loser' Robert De Niro and blasts his acting after star ranted that speech blasting former president was cut from award show teleprompter

On Thursday, the Biden administration announced that it had worked with drug companies to release an additional 230,000 doses of the drug, expected in January.

The flu is also affecting children: So far this season, 12 children have died, the CDC has reported.

Emergency room visits for cases of pneumonia in children have also increased since September, the CDC said, though the number of those illnesses is what would normally be expected during cold and flu season, and is no more severe than usual.

What vaccines are recommended this winter?

According to the CDC, everyone 6 months and older should get the flu and Covid vaccines. Some people, depending on their previous vaccination status or health, might need two shots this season.

Research shows that it is safe to get both shots at the same time.

New this year, people age 60 and older have access to a vaccine to help prevent RSV. The vaccine is also recommended for women in a specific window of pregnancy: 32 through 36 weeks gestation.

People can find where Covid and flu vaccines are available near them at Vaccines.gov.