The optimal vaccination schedule to protect pregnant women against Covid-19 is to be explored in a UK clinical trial researchers hope will also allay concerns about getting the jab.

Last week, Prof Jacqueline Dunkley-Bent, chief midwifery officer for England, urged expectant mothers to get vaccinated as soon as possible, with evidence suggesting the Delta variant poses a significantly greater risk to pregnant women than previous forms of the virus.

A clinical trial called Preg-CoV has been launched to help determine the best gap between doses for pregnant women as well as exploring in greater detail potential side-effects and the impact on babies – something the researchers hope will offer reassurance.

“Pregnant women do have an increased risk of severe Covid-19, of hospitalisation, of intensive care admission, of death, and they have an increased risk of delivering prematurely, compared to pregnant women without Covid-19,” said Paul Heath, chief investigator of the trial and professor of paediatric infectious diseases at St George’s, University of London.

“For that reason we really do need to make sure that when we are vaccinating pregnant women we are doing so in the most optimal way to ensure they are best protected.”

Asma Khalil, lead obstetrician for the trial and professor of obstetrics and maternal fetal medicine at St George’s said that while the UK Covid vaccination programme had been a success, uptake has been low among pregnant women.

According to research in her own hospital, “among pregnant women who’ve given birth between March this year until beginning of July, less than one third – 28% – of women who were eligible according to the guidance actually received [at least one dose of] the Covid vaccine” during pregnancy, said Khalil.

That is despite no safety concerns being raised by real-world data from more than 130,000 pregnant women in the US and 52,000 in England.

“Pregnant women are still concerned because pregnant women were not included in initial Covid vaccine trials,” said Khalil.

In the first phase of the Preg-CoV trial, the team hope to recruit 600 pregnant women, aged between 18 and 44, from 13 sites across England.

Two groups of 200 unvaccinated pregnant women at different gestation times will be randomised both with respect to whether they receive a Moderna or jab and to whether they are given their second dose four to six weeks or at 8-12 weeks after the first dose.

Participants will not know which Covid jab they are given, added Heath, while a routine vaccination to protect against whooping cough will also be included in the schedule so that participants are not aware which dosing regime they are following.

A third group of about 100 pregnant women will be given one dose of a Covid jab at 28-34 weeks gestation, with the second dose of the same vaccine given after delivery, while the fourth group of 100 women will already have had their first dose of any Covid jab before or very early in pregnancy and will get the second dose of the same vaccine.

All the women will have follow-up visits and blood tests, and fill in an electronic diary to help the researchers monitor any potential vaccine side-effects. The team will also track outcomes for the babies up to 12 months of age to explore safety and impact on their development.

However, experts note there is no evidence of harm to babies: rather, the jabs are beneficial as they reduce the chances of pre-term birth or stillbirth, while antibodies can cross the placenta, helping to protect the child against Covid.

Heath said the trial would “fill in the gaps” in current knowledge about vaccinating pregnant women with the Covid jabs. The first results on potential side-effects are expected towards the end of the year and on immune response data in the first quarter of next year.

“I think there will be some lessons learned from this pandemic,” said Khalil. “And one of them is that we should consider including pregnant woman at a relatively early stage for vaccine trials.”