British health authorities urged pregnant women to receive the vaccine against the coronavirus, after a national study showed that the Delta mutant appears to double their risk of severe symptoms.

The official obstetrician in England urged pregnant women to receive the vaccine, following the release of new data that showed an increase in severe symptoms among pregnant women who were hospitalized with symptoms of the virus.

Jacqueline Dunkley Daughter has written to GPs and midwives, urging them to encourage pregnant women to get vaccinated.

She said she called on pregnant women to "protect themselves and their children."

The Royal College of Obstetricians and Gynecologists and the Royal College of Midwives made similar recommendations for vaccinating pregnant women.

Public Health England said they recommend immunizing pregnant women with the vaccines Moderna and Pfizer because they have been given to more than 130,000 pregnant women in the United States.

A study based on national data collected by the UK Obstetric Surveillance System and published online July 25 found that the proportion of pregnant women admitted to hospital with moderate to severe symptoms rose "significantly" after the Delta mutant became dominant in May.

The study, prepared by researchers from Oxford University, found that pregnant women who were hospitalized during the Delta wave were more likely to develop pneumonia, and a third of them needed respiratory assistance.

"It is worrying that hospitalizations of pregnant women with Covid-19 are increasing, and it appears that pregnant women are more affected by the mutated version of the virus Delta," said study leader Marian Knight, professor of maternal and child health at Oxford University.

None of the 3,000 pregnant women who were hospitalized with symptoms of the virus since February were fully vaccinated, according to the study.

The study did not include pregnant women who developed mild symptoms and were treated outside the hospital.

WHO Director of Vaccines Kate O'Brien said this week that pregnancy has been shown to increase the risk of serious symptoms.

"It has a higher risk and is more likely to apply later in pregnancy when the abdomen is large and lung capacity is reduced ... It is really important for pregnant women to realize the importance of vaccination," she said during a question and answer session on social media.

The United Kingdom has recommended since April that pregnant women be vaccinated, but the response has been very slow compared to the general population, according to the study, and this was blamed on misinformation, exacerbated by changing recommendations at the beginning of the vaccination campaign.

"The results of the study strongly highlight the urgent need for an international approach to address this misinformation and encourage vaccination during pregnancy," the report said.

A study conducted by the Royal College of Obstetricians and Gynecologists in May showed that 58 percent of pregnant women who were offered the vaccine refused it, and the majority said they feared harming the baby or were waiting for more information on its safety level.