The way a newborn is delivered could change the way their immune system later responds to life-saving medicine.

A new study from China suggests the route from womb to world, whether vaginal or surgical, can impact how well the measles vaccine works. Children born via cesarean section need that second jab more than most, researchers found. Without that booster, the vaccine is at a higher risk of failing.

The analysis was conducted by scientists at the University of Cambridge and Fudan University in China and includes data from 1,505 mother-infant pairs in China.

Typically, about 5 percent of children given the first dose of the measles vaccine under the age of 1 do not show an antibody response. Between 2013 and 2018, however, babies born via C-section in China were 2.56 times more likely to experience measles vaccine failure than those born vaginally.

Luckily, the second measles jab made up for that failure, triggering a belated and "robust immune response". But that lag is important to know about, as it can help inform effective vaccine policies.

In 2021, millions of children around the world missed their measles booster - a record number that puts herd immunity in numerous nations at a dangerous tipping point.

"We know that a lot of children don't end up having their second measles jab, which is dangerous for them as individuals and for the wider population," explains geneticist Henrik Salje from the University of Cambridge​.

"Infants born by C-section are the ones we really want to be following up to make sure they get their second measles jab, because their first jab is much more likely to fail."

Measles is a viral respiratory disease of imminent threat and one of the most contagious diseases we know of. The vaccine is all we have to reign it in, and it requires at least a 95 percent vaccination rate to achieve herd immunity in a population.

China is close to eliminating measles, but in recent outbreaks, roughly a third of those falling ill with the virus were already vaccinated. These 'breakthrough' infections happen when an individual's immune system either fails to respond to a vaccine, or responds too weakly.

A certain percentage of breakthrough infections are expected, which is why getting as many people vaccinated as possible is so important, but this is a delicate balance, and one that a rise in C-sections could throw out of whack.

Children born via C-section, as opposed to those born vaginally, have a slightly higher likelihood of some immune disorders, and while scientists still cannot work out why that is, there's every reason to dig further.

In 2022, a study linked C-sections to lower antibody responses after meningococcal and pneumococcal vaccines. In the research, babies born vaginally had double the level of protective antibodies after receiving the jabs.

At the time, scientists theorized that babies born via C-section are not 'seeded' with the same important germs from their mother's vagina, and these can have a stimulating effect on a newly formed immune system.

The study in China did not explore why C-sections impacted vaccine responses, but the authors suspect a similar cause.

"With a C-section birth, children aren't exposed to the mother's microbiome in the same way as with a vaginal birth," explains Salje.

"We think this means they take longer to catch up in developing their gut microbiome, and with it, the ability of the immune system to be primed by vaccines against diseases including measles."

For now, however, that is just a theory. While C-section babies do show a different range of bacteria in their guts compared to those born vaginally, other studies suggest the differences disappear after about 9 months.

The newborn immune system is largely a mystery. Researchers can't even agree on whether a baby is born sterile into the world, or already equipped with a microbiome from the womb. It's also unclear whether it is the mother's vaginal microbiome or intestinal microbiome that is seeded in a newborn, and how much breastfeeding plays a role after birth.

Since 2000, the rate of C-sections has doubled worldwide. The surgery now accounts for about 20 percent of births globally, and up to half of all births in some countries like Brazil.

It is crucial for public health that we know how the popular procedure is impacting the immunity of the next generation.

The study was published in Nature Microbiology.