The amount of influenza–specific antibodies present in an individual’s blood can indicate not only if they experienced the flu, but potentially when – a finding that could improve disease monitoring in the tropics, where flu season is unending.

In the largest study of its kind, an international team, led by researchers from the Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam, the Erasmus Medical Centre in Rotterdam, Netherlands, and Penn State University, identified antibody concentrations that correspond to recent and past exposure to the flu strain H1N1.

A paper describing the research was published in a July issue of the journal Scientific Reports.

Approximately one month after infection, the number of flu–specific antibodies in the blood begins to decrease, but some antibodies continue to circulate long after the virus has cleared. In the past, scientists have measured the concentration of antibodies remaining to identify whether an individual has been exposed to the virus, but the results of these tests have typically been limited to describing the presence or absence of past infection.

“In this study we showed that there is a lot more information in measurements of antibody concentration than just presence or absence,” said Maciej Boni, associate professor of biology at Penn State and a lead author of the paper. “Our results show that antibody concentration should be able to provide information about the timing of past influenza infection.”

This information is especially valuable in tropical climates. “In temperate regions like the United States, we might collect blood samples when the flu season is over to see what percentage of people were infected during that flu season,” Boni explains. “But in the tropics there is no flu season — it may be constantly circulating or it could come in waves. If all you measure is the presence or absence of antibodies, you can’t determine when those individuals were infected."

The research team analyzed over 20,000 blood samples from four hospitals in southern Vietnam, taken every two months between 2009 to 2013.

“This is the largest study of its kind, and custom statistical methods needed to be developed for this analysis,” said Nguyen Thi Duy Nhat, graduate student at Oxford University Clinical Research Unit at the time of the study and first author of the paper. This immense undertaking will allow the team to map out the H1N1 flu strain’s dynamics in the tropics in the next phase of their research.

The research team defined four categories of H1N1–specific antibody concentrations. The highest concentrations indicate exposure to H1N1 within the last six months, the second highest concentrations indicate exposure greater than six months prior, and the lowest two categories of concentrations indicate no previous exposure to the virus. Use of these categories could allow public health officials in other tropical locations to determine infection rates of H1N1 with systematic sampling, for example, by screening a subset of the population every January to determine the previous year’s infection rate.

The researchers used a protein microarray to measure antibody concentrations. Developed in the Netherlands, this relatively new technique allows precise antibody measurements with very small volumes of blood.

"This protein microarray has high reproducibility and can provide specificity to 16 different influenza strains,” said Erwin de Bruin, senior laboratory technician at Erasmus Medical Centre and an author of the study. “The small volume of blood required provides a simpler way to perform large epidemiological studies.”

“This microarray, and the additional information about time of infection from antibody concentrations, could change how we monitor disease in the tropics,” added Boni.