Kaohsiung Medical University develops blood test method to identify asymptomatic breakthrough infections

A Kaohsiung Medical University research team has identified antibody changes that may reveal COVID-19 infections people never knew they had.

The university’s Center for Metabolic Diseases and Obesity Research tracked 129 people with no recorded history of infection who had received different COVID-19 vaccine combinations. The researchers compared anti-S and anti-N antibody levels and measured the participants’ ability to neutralize the virus.

Some participants had no diagnosis or symptoms, yet their anti-S antibody levels six months after a third vaccine dose were higher than they had been two weeks after the dose. Researchers said the unexpected rise became a key clue in investigating asymptomatic breakthrough infections. In general, anti-S antibody levels decline over time after vaccination. A sustained or renewed increase may indicate that the immune system encountered the virus again.

The team used longitudinal tracking, ELISA testing, and both pseudovirus and live-virus neutralization tests. It identified three antibody patterns: confirmed infections showed high anti-S and anti-N levels; uninfected participants showed low anti-S levels and baseline anti-N levels; and asymptomatic breakthrough infections showed medium anti-S levels with anti-N remaining at baseline.

Among the participants, about 21% had confirmed infections, 17% had asymptomatic breakthrough infections and 62% had not been infected. The researchers said studies that divide people only into confirmed and unconfirmed cases could miss the third group and underestimate the actual infection rate.

The university said the study aimed not simply to prove that asymptomatic infections exist, but to offer a way to identify them as vaccination becomes widespread. The team has applied for a patent for the method. The findings appeared in the 2026 issue of the Journal of Biomedical Science.