Unvaccinated pregnant women account for nearly 20 percent of the most critically ill coronavirus patients requiring lifesaving care in England in recent months, according to the country’s National Health Service.

“Since July, one in five covid patients receiving treatment through a special lung-bypass machine were expectant mums who have not had their first jab,” the NHS said in a statement Monday.

Out of all women between the ages of 16 and 49 being treated with a therapy called Extracorporeal Membrane Oxygenation — used only when a patient’s lungs are so damaged by the virus that a ventilator cannot maintain oxygen levels — pregnant women make up almost a third, up from just 6 percent at the start of the pandemic.

NHS England released the statistics as part of a renewed effort to persuade pregnant women to get fully vaccinated, amid concerns that anti-vaccine campaigners are fueling unfounded fears that covid-19 vaccinations could harm the mother, baby or both. Britain began administering the vaccine to pregnant people in April.

In the United States, hospitals are also encountering a wave of severe illness and death among young, pregnant and unvaccinated people. Just 26 percent of pregnant Americans have received at least one dose of the coronavirus vaccine while expecting, according to data from the U.S. Centers for Disease Control and Prevention as of Oct. 2, a figure that places them among the nation’s most hesitant populations.

Their reluctance appears to be driven partly by months of unclear guidance and a lack of data for the expectant, both of which persisted until this summer — a consequence of their exclusion from early coronavirus vaccine trials, The Washington Post has reported.

The medical establishment has now fully endorsed receiving the coronavirus vaccine during pregnancy. The CDC formally recommended it on Aug. 11, after long-awaited studies showed no increased risk of miscarriage. Leading obstetrics and gynecology organizations unequivocally backed vaccination in late July.

England’s top midwife on Monday emphasized that the vaccine is safe and effective during pregnancy and is recommended by doctors.

“This is another stark reminder that the covid-19 jab can keep you, your baby and your loved ones, safe and out of hospital,” said Jacqueline Dunkley-Bent, the chief midwifery officer for England.

The World Health Organization has previously warned that pregnant people are at a higher risk of severe cases of covid-19, the disease caused by the virus, compared with others in their age group. The CDC has warned that having covid-19 increases the risk of preterm birth.