Children infected with COVID-19 maintained strong immunity against the virus for at least 18 months, according to a preprint study that was released on Wednesday.

The study, which has not yet been peer-reviewed, was conducted by Kahn-Sagol-Maccabi (KSM), the research and innovation center of Maccabi Healthcare Services in Israel, and was released on the preprint server MedRxiv.

The largest real-world observational research examining children’s immunity to date, it analyzed the Maccabi Health Services records of around 300,000 unvaccinated youth between 1 July  and 13 December 2021, when Delta was dominant in Israel.

Recovered children had to have been infected at least 90 days’ prior to inclusion date to capture reinfections.

According to the research, children and teenagers aged five to 18 who caught COVID-19 developed strong natural protection against reinfection with effectiveness levels of about 80% for at least 18 months.

‘Robust and long-lasting’

“Naturally acquired immunity in children and adolescents was found to be robust and long-lasting, which aligns with what we have witnessed in our day-to-day clinical practice,” said KSM head Dr Tal Patalon, who is both a physician and a researcher, and has been treating COVID-19 patients since the early days of the pandemic.

Although the researchers have not yet looked into natural immunity resulting from Omicron, Patalon said that she assumed “naturally acquired immunity will remain significant against the Omicron variant, this is currently under further research”.

Effectiveness of naturally acquired immunity against recurrent infection reached 89.2% three to six months after first infection, mildly declining to 82.5% nine months to one year after infection and then remaining steady for up to 18 months.

The protection was even stronger for the younger cohort; children up to age 11 showed no significant decline in protection during the study period, while those between 12 and 18 showed more decline.

The study was published just days after the US Food and Drug Administration and the Centers for Disease Control and Prevention approved vaccines for children under the age of five and as cases are on the rise throughout much of the Western world.

The study findings were validated twice, using two different statistical methods, strengthening the validity of the study results.

“As for public health policies, the demonstrated long-term protection of naturally acquired immunity has important implications regarding the decision to vaccinate convalescent children and adolescents, and to mandate self-quarantine after exposure, affecting all biopsychosocial aspects of life and well-being of children, adolescents and their families,” the study’s authors concluded.

They noted that this should be considered in light of evidence of increasing seroprevalence antibodies that indicate previous infection in as much as 70% of children and adolescents in the US.

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