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Children under the age of five had higher levels of binding antibodies than adults after COVID-19 infection: study

The findings from a new study suggest that young children may have a stronger antibody response than adults do after a mild COVID-19 infection.

The study, conducted by researchers at the Johns Hopkins Bloomberg School of Public Health in collaboration with the U.S. Centers for Disease Control and Prevention, looked at samples taken from 682 children and adults in Maryland between November 2020 and March 2021.

They found that in 11 out of 12 households where multiple residents had previously had COVID-19 and at least one child was aged four years or younger, the child had the highest levels of binding and neutralizing antibodies out of everyone in the household.

“This study demonstrates that even children in the first few years of life have the capacity to develop strong antibody responses to SARS-CoV-2 infection, which in some cases exceed adult responses,” Ruth Karron, a professor in the Department of International Health and director of the Johns Hopkins Vaccine Initiative at the Bloomberg School and lead investigator of the study, said in a press release.

The findings were published Tuesday in the journal JCI Insight.

Participants were sourced through a household surveillance study aiming to learn about COVID-19 infection in children under the age of five — those included in the study were parents and households that contained at least one child who fell into this age group and agreed to around eight months of follow-up to track for COVID-19 infection among the household.

Out of the 682 people included, blood samples revealed that there were antibodies present in 56 people from 22 households, indicating prior COVID-19 infection.

Fifteen of these 56 were children under the age of five, 13 were children aged five to 17 years, and 28 participants were adults 18 years or older.

The youngest participant found to have antibodies was just three months old.

Researchers looked at not just neutralizing antibodies, but a specific antibody that targets the “receptor-binding domain” (RBD) on SARS-CoV-2’s outer spike protein, blocking the virus from binding to human cells.

They found that these binding antibodies were at much higher levels among the children than the adults.

The levels were more than 10 times higher in children aged under the age of five, and almost nine times higher in children five to 17.

In terms of the levels of neutralizing antibodies as a whole, they were still nearly twice as high in the children under four (not four and under?) when compared to the adult levels.

None of the participants with a suspected prior COVID-19 case had to be hospitalized due to the virus. Around half of those who had binding antibodies had not had a COVID-19 case confirmed by a health professional, suggesting many milder cases may go uncounted.

The study looked at antibody response in reaction to infection, not to vaccination, but since vaccines are designed around the spike protein, it still can speak to the effectiveness of vaccines, researchers said.

“Very young children in our study developed high titers of antibody to the SARS-CoV-2 spike protein, which is the target antigen for COVID vaccines,” Karron said in the release. “These findings should provide some reassurance that with the appropriate vaccine doses we can effectively immunize very young children against SARS-CoV-2.”

How COVID-19 impacts children is still one of the areas where knowledge is lacking.

Not many studies have compared adult antibody responses to the antibody response of children. The release pointed out that a previous study of hospitalized patients found that adults produced a higher level of neutralizing antibodies, but community-based studies of milder cases support the finding that children do mount robust immune responses.

A recent U.S. study published earlier this week looking at antibody levels in children in Texas found that around 97 per cent who tested positive for COVID-19 still had antibodies up to seven months after they were initially assessed.

Researchers pointed out that children appear to have much higher levels of binding antibodies, which could play into why children in general are more likely to have milder cases of COVID-19.

“It is possible that these robust humoral immune responses diminish rates of serious or highly symptomatic infection by promoting viral clearance,” the study stated.

Researchers noted that the study is limited by its small scope in terms of how many households contained individuals with antibodies. The timing of the study also means that many of the responses are from infection with the original strain of COVID-19, meaning results may not be directly applicable to all other variants. 

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