When Leila Sawenko and her husband Tony Maguire reunited with their children after a week away in Sydney in March 2020, lots of cuddles and kisses were shared. Unknowingly, the couple also shared something else with their three young children – COVID-19.

A week after arriving home, Leila and Tony began to feel unwell. Leila received an email saying some NSW relatives had tested positive for the virus, prompting the couple to get tested. The results came back positive.

The couple's three children – Bodhi, 9, Lenny, 7, and Marley, 6 – were tested but negative. It was a surprise, given they lived in such close quarters with their parents.

Technically, this was a case of chronic exposure. So, what was going on? Suddenly, the family became a perfect case study for researchers trying to understand how children's immune response to COVID-19 differed from adults.

The family of five shared blood, saliva, stool and urine samples, as well as nose and throat swabs, with a team of 30 researchers led by the Murdoch Children's Research Institute (MCRI). Samples were taken every two to three days over a month.

While Leila and Tony experienced heavy flu-like symptoms, their children's experience was more like a cold. Bodhi reported a cough, sore throat and abdominal discomfort, while Lenny had a mild cough and runny nose. Marley, the couple's youngest child, who had been sharing a bed with her parents while they were infectious, displayed no symptoms at all.

"It was jaw-droppingly amazing because they'd spent a week and a half with us while we were COVID-positive," Leila says. The children's initial negative test results baffled MCRI paediatrician Dr Shidan Tosif, who tested them a further three times – even using two different laboratories. But further studies revealed they had all contracted COVID-19.

Shidan says it was striking that all three children were found to have salivary antibodies to the virus. The one with the most robust immune response was Marley, who was asymptomatic.

"The fact these children were able to shut down the virus without even showing a positive test result suggests they have some level of their immune system which is able to respond and deal effectively with the virus," he says.

In one of the first studies of its kind, MCRI researchers established that COVID-19 was unlikely to have severe or lasting effects on younger age groups because children's immune systems responded differently to those of adults.

At a stage of the pandemic when so much was unknown, it was reassuring news for parents, carers and researchers alike.

The findings cemented MCRI's reputation as one of the top children's immune research institutes globally and armed researchers with clues as to why children can shut down the virus so effectively.

This led to the Institute launching a larger study, which has grown to become one of the world's most in-depth examinations of children's immune system responses to COVID-19. Almost 50 children and 70 adults from 28 Melbourne households infected with or exposed to COVID-19 are participating.