woman with covid mask

Hervé, a father of two in Paris, was shocked when he found out that his daughter, Cèlia, had been infected with Covid-19. He, Cèlia, Cèlia’s mother and brother had multiple negative PCR tests since the pandemic began and had never felt sick.

But in January 2022, when Hervé took Cèlia, 10, to a pediatric vaccination clinic at Robert Debré Hospital, one of the doctors asked permission to do a quick fingerstick blood test on Cèlia for a medical research study before administering her shot. Hervé and his daughter agreed.

“During [the past] two years, we didn’t notice any symptoms or anything related to Covid symptoms,” he said in a telephone interview, speaking on condition that the family’s last name not be used to protect their privacy. “So we were pretty confident that Covid had really avoided the house and the family.”

After 20 minutes of waiting, the doctor brought them back to the exam room. Cèlia’s antibody test came back positive, meaning that her immune system had fought off the virus at some point in the past. There was no way of knowing how or when she was infected. The test could only tell whether or not Covid antibodies were present, not when they arrived.

“It was a pretty shocking surprise,” Hervé said. “This is a very strange situation when a doctor gives you the information that your daughter had been sick, but you didn’t notice anything.”

Case numbers are undercounted

Cèlia is not alone. A global meta analysis from Peking University in Beijing, China, estimated that over 40% of people infected with Covid had no symptoms. Dr. Jillian Hurst, a research director and assistant professor of pediatrics at Duke University, said that in her research, about one in three children with Covid were asymptomatic.

The World Health Organization has confirmed almost 500 million Covid cases since the beginning of the pandemic, with 1.7 million new cases just in the last 24 hours. But based on her study and other data on asymptomatic patients, Dr. Hurst thinks health authorities may be undercounting the actual number of people infected.

“What that said to me is, we probably have more positives out there than we think,” Hurst said in a phone interview.

While Covid-positive individuals without symptoms may not be at risk of severe illness in the short term, life-threatening complications can occur down the line.

“We have seen kids who developed MIS-C after asymptomatic Covid,” Dr. Hurst said. MIS-C, Multisystem Inflammatory Syndrome in Children, is a potentially deadly condition that can develop in children after Covid.

“[Children] will turn up with cardiac complications six weeks later, and then they will be antibody positive, but nobody remembers them having had an actual infection,” she said. “So to me that says that even if you have an asymptomatic infection, there could be long-term immune complications.”

Early research on long Covid, a debilitating post-viral condition affecting millions of people in the U.S. alone, indicates that almost a third of those affected were either asymptomatic or had mild symptoms.

Individual health outcomes are not the only danger leaving asymptomatic cases out of the count. While there is still debate among researchers exactly how contagious asymptomatic people are, it is generally accepted that they are a significant source of community spread.

Future immune protection

While scientists are still working to build a picture of what it means to have an asymptomatic case of Covid, Dr. Hurst’s early data has at least one encouraging finding.

“[Before the study], we didn’t know if kids would mount an effective immune response if they were asymptomatic,” she said. “But we didn’t see any differences in kids who had asymptomatic versus symptomatic infection.”

That means that the children who had recovered from asymptomatic Covid had just as much post-illness immune protection as the children with mild or moderate symptoms.

Virginia Jeffries

Virginia Jeffries is a journalist in New York City. Since 2020, she has reported on long...