SciTech

Early flu exposure generates lifelong effect on immunity

The H1N1 (left) and H3N2 (right) flu viruses, depicted using electron microscopy. Researchers found that exposure to both strains at a young age improves future immunity, but the body fights them differently. (credit: Courtesy of National Institutes of Health via Wikimedia Commons) The H1N1 (left) and H3N2 (right) flu viruses, depicted using electron microscopy. Researchers found that exposure to both strains at a young age improves future immunity, but the body fights them differently. (credit: Courtesy of National Institutes of Health via Wikimedia Commons)

In 2016, scientists reported that exposure to influenza viruses during childhood can give people partial protection against distantly related influenza strains for the rest of their lives. The idea that past exposure to the flu virus determines a person's response to future infections, also known as "immunological imprinting,” overturned the belief that exposure to flu virus produced no immunological production. This sparked a series of interesting yet puzzling findings by researchers.

Two subtypes of influenza virus, H1N1 and H3N2, are common types of flu that have caused a number of seasonal outbreaks over the past decades. H1N1 is more likely to affect young and middle-aged adults. H3N2 is more dangerous than H1N1, considered high risk for the elderly, leads to more hospitalization than H1N1, and causes the majority of deaths from flu. Every year, H3N2 alone kills an estimated 36,000 people in the U.S. According to health record data, exposure to the diseases at an early childhood stage would generate a lifelong impact to a child’s immunity.

According to the Arizona Department of Health Services, people first exposed to the less severe strain, H1N1, during childhood are less vulnerable to H1N1. They are less likely to be impacted by H1N1 if they encounter it later on in their lives compared to people who were first exposed to H3N2. At the same time, people first exposed to H3N2 received extra protection against H3N2 later in life.

By examining the evolutionary relationships of the two strains of flu, scientists learned that H1N1 and H3N2 belong to two separate branches on the influenza family tree. This means that if an immune system has fought one of the two diseases, it cannot fight the other disease as easily because the other disease is new to the immune system.

However, medical records also revealed that people whose first childhood exposure was to H2N2 did not have a protective advantage when they later encountered H1N1, even though H2N2 and H1N1 are close relatives to each other. This indicates the difficulty for our immune system to recognize and defend against some closely related strains of diseases, particularly the flu. Scientists realized that our immune system response applies to some strains the same, but others quite differently.

Currently, scientists are studying immunity against bird flu due to the breadth of our immune system response to those strains, hoping to apply their discoveries to fighting seasonal flu, which has a more severe impact on the general population. Researchers also hope their findings could help predict which age groups might be severely affected during future flu seasons based on the subtype circulating. With this information, we could find ourselves in a future where health officials can prepare their responses allowing people to receive personalized vaccines available in limited quantities.