Children may carry the coronavirus for longer than adults and for longer than previously thought, a new study in the United States has found.
Researchers at the Children’s National Hospital in Washington analysed the medical records of more than 60 children who had been tested multiple times for Covid-19, the disease caused by the virus, and found the median time it took to test negative was 25 days.
That was longer than some previous estimates. A recent study by the National Clinical Research Centre for Infectious Diseases in Shenzhen, China, for instance, found most children were clear of the virus after 18 days.
“The takeaway here is that we can’t let our guard down,” said Burak Bahar, lead author of the new study published in the Journal of Pediatrics, in a statement on Thursday.
Among adults, the median time from testing positive to testing negative is around 20 days, according to studies from around the world.
While it is rare for people to be passing on the virus by shedding particles for longer than a month, there have been extreme cases of this happening, such as an asymptomatic patient in southwest China who only tested negative after 45 days.
Bahar’s team found one of the children at the Washington hospital tested positive 62 days after the infection was first confirmed. They noted it was “rare” to conduct serial testing on children – this only happened in about one in 10 cases in the US, according to the medical records examined for the study.
They also said testing positive did not necessarily mean that a child recovering from the virus was still passing it on, since the result could be caused by gene fragments. They said further studies on whether it was possible for the virus to replicate from these fragments were under way.
Much is still unknown about how the virus affects children, or why they are less likely to get infected and develop severe symptoms than adults.
The Shenzhen team, for instance, found that nearly half of the 35 children in their study had no symptoms at all. Some had infections in both lungs, but apart from fevers and mild coughs, none of them were in a serious condition.
One theory as to why children appear to be less vulnerable to the virus is that they might have less ACE2 – a protein to which the coronavirus binds in order to enter a host cell. Other researchers have suggested that children’s immune response to the virus could differ from that of adults. But there is no scientific evidence as yet to prove either of these differences.
Bahar’s team also looked at the time to seropositivity, or the presence of antibodies in the blood. The median time to develop antibodies was 18 days for children – slower than adults, who usually had antibodies in 10 to 15 days, according to previous studies.
The researchers estimated it would take children a median time of 36 days to build up enough antibodies to beat the coronavirus, but about half of those in the study did not reach that level in the time between their first and last tests.
“With most viruses, when you start to detect antibodies, you won’t detect the virus any more. But with Covid-19, we’re seeing both,” Bahar said. “This means children still have the potential to transmit the virus even if antibodies are detected,” she added.
Previous studies have suggested that the illness is more likely to be worse in older children. This was not seen in the Washington study, which instead found that female patients aged from 6 to 15 took longer to clear the virus (a median of 32 days) compared to those aged 16 to 22 (18 days).