Sarah wasn’t too worried when her two-year-old son contracted Covid in October last year. “I was completely naive. Everyone says children are not that affected by it, so I just thought he was going to have a bad sort of cold and bounce back,” says Sarah, 40, from Northamptonshire.

Initially, he had a high temperature and a heavy cold. Then, nine days after testing positive, his condition became more serious. “I noticed when he was breathing, he was pulling his tummy in under his ribs, which shows you’re struggling to breathe. I thought, ‘Oh God, that’s not good’.”

Her son was taken by ambulance to hospital. There, doctors said his oxygen level was too low, his temperature too high, and his heart rate too high. Doctors fitted a nebuliser to his nose, to help him get oxygen. The treatment worked. After a few hours, he was allowed to go home. A few days later he “turned the corner” and his condition improved.

But last week, he tested positive for Covid again, for the second time in four months. This time he probably has the omicron variant (whereas last time he probably had the delta variant). So far, his symptoms are much milder this time around – mostly headaches and a runny nose. But it’s causing Sarah understandable stress.

Sarah is now keen to get her son vaccinated, once it is medically approved. She says she would “absolutely” sign up for a jab for him, as soon as the data shows it to be safe and effective.

Sarah’s case is rare: most infants infected with Covid have mild symptoms, and most are not admitted to hospital. But still, she is one of many parents across the UK growing impatient with the rules around vaccinating children. Currently, anybody aged over 12 is eligible for a full Covid vaccination (the same dose as given to adults), while five to 11-year-olds are only eligible if they are clinically vulnerable, or if they live with someone who is immunocompromised.

Those aged below five, on the other hand, are not eligible for vaccination under any circumstances, even if they were born with underlying conditions. Parents like Sarah are starting to wonder why.

Most countries do not yet vaccinate under-fives, but there are some exceptions. Israel is expected to roll out a special dose of the Pfizer vaccine to babies as young as six months, starting in April, according to comments last week from a senior Israel health minister. China has approved a Sinovac vaccine (not offered in the UK) to children as young as three. In late November, Hong Kong also lowered the age limit for the Sinovac vaccine to three.

Their argument received new impetus last week after Sage released new data indicating an increase in the proportion of infants being admitted to hospital with Covid – though most are not becoming seriously ill. Since mid-December, infants aged below one have made up 42 per cent of paediatric admissions with Covid. In earlier waves, when other variants were dominant, they made up only about 30 per cent.

For most of the population, the omicron variant appears milder than previous variants. But babies aged below one look like the one exception, explains Russell Viner, professor of child and adolescent health at University College London. For infants, the case admission rate (the share of infected people who end up in hospital) is “very slightly higher” with omicron than with previous variants, he says.

Experts note that Covid is currently widespread among children (about one in 12 under-18s is currently infected) and so we should expect to see some hospital admissions. Many infants being taken to hospital are probably being taken for an unrelated reason, “like an in-grown toenail”, says Viner, then diagnosed with Covid by chance while on the ward.

But it also might be explained by the way omicron interacts with our body. While previous variants of Covid have tended to inflame the lower parts of the airways (particularly the lungs), omicron tends to attack the upper respiratory tracts (particularly the throat and windpipe). For adults and older children, this normally means a milder disease. But because babies have very small airways, an inflamed respiratory tract can in rare cases cause difficulty breathing.

It may also be explained by immunity: young infants have low levels of Covid immunity, probably because they haven’t been vaccinated and are less likely than older children to have been exposed to the virus. Hospital policy may also be a factor. The NHS tends to operate a “precautionary stance” towards babies. “When you’re a baby, NHS 111 is much more likely to say: ‘Come to hospital’, whereas for an adult with the same symptoms, they’ll say: ‘Wait and watch’,” says Viner. 

Importantly, Viner and other experts stress that most of the babies being admitted to the hospital for Covid do not have a serious illness. “It appears that a lot of these babies that are coming in with omicron are only been admitted for 24 hours or 48 hours for observation. Many of them are having no treatment and they’re going home.”

Alasdair Munro, paediatric registrar and clinical research fellow in paediatric infectious diseases at University Hospital Southampton, points to an FT report that says although rates of omicron have risen, it is nowhere near what clinicians see in a normal year for usual respiratory bugs in children.