Adults may be more resistant to bird flu virus than children

The human immune system constantly adapts to the flu viruses it encounters. Throughout life, exposure to different flu strains builds a complex immune memory. Recent research suggests that this memory might help protect against emerging threats like the H5N1 avian influenza virus.

Scientists at the Perelman School of Medicine at the University of Pennsylvania have discovered that people exposed to certain flu viruses in childhood may have some level of immunity against H5N1.

Their findings, published in Nature Medicine, reveal that older adults, particularly those exposed to flu strains before 1968, possess antibodies that can recognize H5N1.

These antibodies may not stop infection but could make H5N1 less severe. The findings show who is at risk and who needs the vaccine most.

Flu exposure in childhood

Scientists have long known that childhood flu infections leave lasting imprints on the immune system.

The body learns to recognize the flu strains it first encounters, creating antibodies that can last for decades. These antibodies are often most effective against flu viruses similar to those encountered in early life.

“We know that early childhood influenza exposures can elicit immune responses that last a lifetime,” said senior author Dr. Scott Hensley, a professor of microbiology.

“We found that antibody responses that were primed by H1N1 and H3N2 viruses decades ago can cross-react to H5N1 avian viruses circulating today. Most of these cross-reactive antibodies cannot prevent infections, but they will likely limit disease if we have an H5N1 pandemic.”

This means that people who were first exposed to H1N1 or H2N2 viruses before 1968 may have antibodies that can recognize the stalk of H5N1’s hemagglutinin protein.

Since the stalk evolves more slowly than the head of the hemagglutinin protein, it remains more similar across different flu strains. However, younger individuals, who first encountered later flu viruses, may not have the same level of cross-protection.

Bird flu virus is changing fast

H5N1 viruses have been present in bird populations for decades, but a newer version, called clade 2.3.4.4b H5N1, has recently spread among cattle.

So far, this virus does not easily infect humans, as it does not bind well to receptors in the upper respiratory tract. However, if it mutates to infect human airway cells more efficiently, it could potentially start spreading between people.

Influenza viruses are named after the two key proteins on their surface: hemagglutinin (H) and neuraminidase (N). These proteins allow the virus to enter and exit host cells.

Current flu vaccines work by training the immune system to recognize the hemagglutinin head, preventing infection.

However, the hemagglutinin head changes rapidly, making it difficult to develop long-lasting vaccines. The stalk of the hemagglutinin protein evolves more slowly, meaning antibodies that recognize it may offer broader protection across different flu strains.

Prior flu exposure and H5N1 immunity

To understand how prior flu exposures influence immunity, the researchers analyzed blood samples from over 150 individuals born between 1927 and 2016. They looked at antibodies targeting the hemagglutinin stalk across multiple flu viruses, including H5N1.

The analysis showed that people born before 1968 had more antibodies capable of binding to the H5N1 stalk. In contrast, younger individuals, who did not encounter earlier flu strains, had lower levels of these antibodies.

A person’s birth year appeared to strongly influence their ability to fight H5N1. Older adults had a greater number of cross-reactive antibodies, while children and younger adults had significantly fewer.

This suggests that younger individuals may be at greater risk if H5N1 were to spread widely among humans.

Vaccines can protect the most vulnerable

To further explore vaccine responses, the researchers examined another group of individuals born between 1918 and 2003.

These individuals received a 2004 H5N1 vaccine, which did not exactly match the clade 2.3.4.4b strain currently circulating. The experts took blood samples before and after vaccination to measure changes in antibody levels.

The results aligned with their earlier findings. Older adults already had some antibodies that could recognize H5 stalks before vaccination. After receiving the vaccine, their antibody levels increased slightly. However, in children and younger adults, antibody levels increased much more significantly.

This suggests that while older individuals already have some degree of cross-protection, younger populations may gain the most from targeted vaccination efforts.

“In the event of an H5N1 pandemic, all age groups will likely be highly susceptible, but it is possible that the highest disease burden will be in children,” said Hensley. “If this is the case, children should be prioritized for H5N1 vaccinations.”

Preparing for a possible outbreak

This study shows why knowing how past flu infections affect immunity is so important. Older adults might already have some protection against H5N1 because of flu viruses they encountered long ago. But younger people, who lack this exposure, could be at greater risk if H5N1 changes and starts spreading between humans.

Flu vaccines are still one of the best defenses, even when they don’t exactly match new virus strains. Scientists are closely watching H5N1 and working on vaccines that could protect more people if an outbreak happens.

Some researchers are also looking into universal flu vaccines. Instead of targeting fast-changing parts of the virus, these vaccines would focus on more stable regions, like the hemagglutinin stalk. If successful, they could provide longer-lasting protection against different flu types and help prevent severe illness from future flu threats.

Bird flu immunity may prevent pandemics

While H5N1 is not yet a major public health threat, its continued circulation among birds and cattle raises concerns.

The possibility of mutations that allow it to infect humans more easily means that preparation is essential. Research like this study helps guide vaccination strategies, ensuring that those most at risk receive protection if an outbreak occurs.

By understanding how flu exposure shapes immunity, scientists can develop better vaccines and public health strategies to minimize the impact of future influenza pandemics.

The study is published in the journal Nature Medicine.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–