Influenza (flu) is a common infectious respiratory illness caused by a virus. Exposure to influenza viruses during childhood gives people partial
protection for the rest of their lives against distantly related
influenza viruses, suggested a new study in the journal Science.
Scientists from UCLA and the University of Arizona analyzed data
from all known human cases of two types of avian influenza - more than
1,400 people in all - and found evidence of previously unrecognized
human immunity against several viruses that circulate in animals but
have not previously circulated in humans. They also discovered that
people born before 1968 are more susceptible to certain viruses, while
people born during or after that year are more at risk for different
strains of the flu.
‘Exposure to influenza viruses during childhood gives people partial protection for the rest of their lives against distantly related influenza viruses.’
The viruses charted by the researchers were H5N1 and H7N9, which
have each affected more than 700 people, mostly in Asia and the Middle
East, making them the flu strains that pose the greatest concern for
emergence from animals to humans. H5N1 and H7N9 also both have shown the
ability - albeit a very limited one - to be transmitted from human to
human, which raises the possibility that a slight adaptation in either
could trigger a pandemic.
Since 2013, when H7N9 was first detected, scientists have been
puzzled by the fact that the two viruses tend to affect distinct age
groups: Children and young adults are more likely to be infected with
H5N1, while H7N9 disproportionately affects older adults.
Several explanations for this phenomenon have been proposed, but the
UCLA and Arizona researchers suspected that the cause was people's
pre-existing immunity, said Katelyn Gostic, a UCLA doctoral student who
is the study's lead author.
The researchers can now predict with reasonable precision whether a
person will have immunity against new influenza strains based on their
birth year, which indicates the seasonal flu virus that was most likely
to have caused their first flu infection during childhood.
"Our findings show clearly that this 'childhood imprinting' gives
strong protection against severe infection or death from two major
strains of avian influenza," said James Lloyd-Smith, a UCLA professor of
ecology and evolutionary biology and the study's senior author. "These
results will help us quantify the risk of particular emerging influenza
viruses sparking a major outbreak."
It is unclear whether that imprinting provides strong enough
immunity to prevent infection altogether, but it substantially reduces
the risk for severe disease, the study reports. People born in
"protected" birth years were much less likely to get sick enough to
visit the doctor, be hospitalized or die from H5N1 or H7N9 infections.
The dividing line for the two age brackets, 1968, was the year of
the so-called Hong Kong flu pandemic, which swept away viruses from a
different genetic group that had dominated seasonal influenza for a
half-century beforehand. People born during and since 1968 are more
likely to have protection against H7N9, which is more closely related to
the 1968 virus than to flu viruses that circulated before. People born
before 1968 are likely to have protection against H5N1.
The study reports that the same methods could be used for any
country with sufficient data to estimate which age groups would have the
highest risk for severe disease in a pandemic.
"These findings challenge the current paradigm, where the entire
population would be immunologically defenseless in a pandemic caused by a
novel influenza virus," Gostic said. "Our results suggest it should be
possible to forecast age distributions of severe infection in future
pandemics, and to predict the potential for novel influenza viruses from
different genetic groups to cause major outbreaks in the human
Gostic added that the projections could be made based on demographic
information and influenza surveillance data that's already routinely
collected by governments and public health agencies, in an effort
organized by the World Health Organization.
"This approach opens new frontiers in the nascent field of
quantitative risk assessment for emerging pathogens," Lloyd-Smith said.
"All of the focus has been on studying properties of the viruses and
ecological circumstances that drive spillover. Those factors are
definitely crucial, but it turns out we can learn a lot about flu
pandemic risk from information about humans, which we've already got."