‘Surprising’ low antibody response in children with mild COVID

A small study comparing immune responses has suggested children may be less likely to produce antibodies than adults after mild COVID-19 infections.
 
The peer-reviewed research, which involved the Murdoch Children's Research Institute (MCRI), the University of Melbourne, and the Peter Doherty Institute for Infection and Immunity, analysed seroconversion among 57 children aged 2–10, and 51 adults with an average age of 37.
 
Those involved in the study, which was published this month in Jama Open Network, were recruited through The Royal Children’s Hospital in Melbourne from 10 May to 28 October in 2020, prior to the vaccination rollout.
 
All the participants were either asymptomatic or had mild illness such as headache, nausea, cough and muscle aches. None were hospitalised due to the virus.
 
The findings of this cohort study suggest that for mild COVID-19, children may be less likely to have seroconversion than adults despite similar viral loads.
 
It found that 37% of children showed conversion to Immunoglobulin G compared to 76% of adults.
 
Nose and throat swabs as well as blood samples were collected during the acute phase of the illness, and the presence of COVID-19 was confirmed via a PCR test, while researchers assessed the level of seroconversion using three separate assays.
 
Associate Professor Paul Licciardi, an immunologist at MCRI and the corresponding author for the study, said a strength of the research is the comparison between adults and children with similar viral loads.
 
‘There wasn’t a lot of data out there for children,’ Associate Professor Licciardi told newsGP.
 
‘We were able to get some insights into the capacity of children to respond to the virus in the context of the Wuhan strain.’
 
While the paper notes a higher Ct value was found for adults, the authors describe the difference in viral loads at baseline ‘was not statistically significant’.
 
Both seroconversion and T-cell responses were considered in the paper.
 
‘Our numbers were a bit small for that analysis but we saw some evidence [that] in the children who didn’t seroconvert, they also didn’t have a lot of T-Cell response compared to the adults,’ Associate Professor Licciardi said.
 
‘We thought that children should respond if they had similar symptom levels, but we didn’t find that – at least for this cohort. We found that it was about half the level [of the adults]. That was a little bit surprising.’
 
The researchers say more work is needed to work out why seroconversion and development of symptoms are less likely in children following infection.
 
One theory relates to the greater effectiveness of the children’s response.
 
‘It gave us some idea that perhaps children were just so efficient in dealing with the virus, particularly with the [original strain] from 2020, that they didn’t really need to mount an active response at all,’ Associate Professor Licciardi said. ‘That has been our line of thinking.’
 
He said further laboratory studies are currently being run which he hopes will confirm whether that theory is correct.
 
The authors also say the research ‘could support strategies to protect children against COVID-19, including vaccination’.
 
However, Associate Professor Licciardi said caution is needed when interpreting the results, with similar tests for later strains likely to generate distinct results.
 
‘The caveat to the study, which we alluded to in the paper, was that that this is from Wuhan infection,’ he said.
 
‘What we believe – and we are doing some studies on this at the moment – is that it is probably going to be quite a different situation compared to Delta or even Omicron.’
 
He said early data from a Delta cohort suggests that there was almost 100% seroconversion in children.
 
‘It’s very different,’ he said. ‘We are still working through the data. Delta of course is associated with much higher viral loads and I think that is probably the key thing that is happening.’
 
As a result, the MCRI immunologist said drawing inferences from the study for the current situation in Australia is ‘a bit tricky’.
 
‘We will have new data emerging over the next few months, so watch this space,’ he said.
 
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