Researchers tracked individuals’ antibody levels after vaccinations and identified four distinct patterns of immune response after the first booster vaccination. Notably, the group that started with the highest antibody levels but experienced a faster decline were infected earlier. People with lower blood levels of IgA(S) antibodies, which protect the nose and throat, were also at higher risk. The findings suggest that monitoring how antibody levels change over time could assist in identifying individuals at greater risk of infection.

Led by scientists from Nagoya University in Japan, the research team measured antibody levels in 2,526 people over 18 months to see how vaccine responses changed between the first vaccination and later booster shots. They developed a mathematical classification system for COVID-19 vaccine responses using long-term tracking and AI-based computer analysis, becoming the first to systematically identify and characterize the “rapid-decliner” group.

The researchers found that immune responses fell into four clear patterns: some people maintained high antibody levels over time (durable responders), others started with strong levels but lost them quickly (rapid-decliners), a third group produced few antibodies that also declined rapidly (vulnerable responders), and the rest fell in between (intermediate responders).

Immunity that peaks early and then drops

According to Shingo Iwami, senior author and professor at Nagoya University’s Graduate School of Science, the results for the rapid-decliner group were surprising. “In spite of their impressive initial immune response, they caught COVID-19 sooner than other groups, while durable responders maintained protection for longer periods. One-time blood tests for IgG antibodies, the antibody type we used for classification, couldn’t detect this risk. Only by tracking changes over months did we see the pattern,” he explained.

A breakthrough or subsequent infection refers to infections that occur after vaccination because the virus overcomes the immune protection that vaccines provide. The researchers found that people whose antibodies declined faster, either because they started low or dropped quickly (vulnerable responders and rapid-decliners), were slightly more likely to get breakthrough infections earlier.

After booster vaccinations, 29% of participants fell into the durable responder category, 28% were vulnerable responders, and 19% were rapid-decliners. The remaining participants showed intermediate patterns. The differences in breakthrough infection rates between groups were modest — 5.2% for durable responders and 6% for vulnerable and rapid-decliners.

Breakthrough infections linked to IgA(S) antibody levels

The study also revealed that participants who experienced breakthrough infections had lower levels of IgA(S) antibodies in their blood several weeks after vaccination. These antibodies protect the nose and throat and are our first line of defense against respiratory viruses.

Importantly, the researchers found a strong correlation between blood IgA(S) levels and nasal IgA(S) levels, suggesting that blood tests can reliably indicate the strength of immune protection in airways. As a result, measuring blood IgA(S) levels after vaccination may help identify individuals at higher risk for breakthrough infection, especially among vulnerable groups.

While these results provide a foundation for future research, Professor Iwami emphasized the importance of identifying the underlying biological mechanisms responsible for the rapid decline in antibody levels in order to develop more effective vaccination strategies. Previous research points to factors such as age, genetic variation, vaccine-specific characteristics, and environmental influences, including sleep habits, stress levels, and medications being taken at the same time.

“This is the first time we’ve been able to clearly group how people respond to COVID-19 vaccines,” Professor Iwami noted. “Identifying the rapid-decliner pattern is especially important — it helps explain why some people may need boosters sooner than others. This could potentially contribute to better, more personalized vaccination strategies. However, whether antibody testing can be used widely depends on cost, accuracy, and if the benefits are worthwhile compared to current strategies. More research is needed to understand its full potential.”