About 291,000 to 646,000 people in the world die from seasonal flu each year, higher than an often-used statistic of 250,000 to 500,000 deaths that is has always tilted toward higher-income countries and is based on data thought to reach back to the 1990s.
An international team from 47 countries, which included experts from the US Centers for Disease Control and Prevention (CDC), published its findings, along with details of how the invesigators made their calculations, yesterday in the latest issue of The Lancet.
The earlier death estimate has been cited by the World Health Organization (WHO) since 2004, with no information on how experts arrived at the figures. Because they didn't reflect the experience of developing countries, the numbers are thought to underestimate the true death burden.
The researchers in the new study said that since then, many countries have improved their vital records and flu surveillance systems, enabling more low- to middle-income countries to estimate deaths from the disease.
Poorest regions, elderly most affected
For the new study, the team estimated excess mortality for flu for 33 individual countries—which make up 57% of the world's population—based on surveillance data from 1999 to 2015. Then they extrapolated the estimates to 185 countries spanning six WHO regions, four World Bank income classifications, and selected age-groups. The team also used data from 14 countries to validate their estimates.
The researchers said the new estimate is higher, because the older numbers probably didn't reflect the experience of some of the hardest-hit areas of the world, underestimating the death burden. They noted, however, that their new estimate may mask the true overall death burden, because the study focused on respiratory deaths related to flu, which doesn't take into account flu's known exacerbating impact on other conditions often listed as causes of death, including cardiovascular disease or diabetes.
Results showed that the poorest regions and older adults carried the greatest flu mortality burden, with people age 75 and older and those living in sub-Saharan Africa having the highest rates of flu-linked respiratory deaths. Deaths were high but slightly lower in Eastern Mediterranean and Southeast Asian countries.
The researchers concluded that their work could help guide countries that want to introduce or expand flu vaccination programs and could be used to help win more support from global health groups looking to increase access to the vaccines in low- and middle-income countries.
Danielle Iuliano, PhD, the study's lead author who is a researcher in the CDC's influenza division, said in a CDC press release, "This work adds to a growing global understanding of the burden of influenza and populations at highest risk. It builds the evidence base for influenza vaccination programs in other countries."
Considerable variation among nations
In a related commentary in the same Lancet issue, Sheena Sullivan, PhD, with the WHO collaborating center and Peter Doherty Institute for Infection and Immunity in Melbourne, Australia, said the study provides a much-needed update to the earlier WHO estimate.
Having a global estimate is useful, she said, but it masks considerable variation among countries, and the country-specific estimates in the new study are valuable, because they can be used by policymakers.
"Often, no mortality estimates are available for low-income and lower-middle-income countries, where limited health-care infrastructure and the competing demands of other communicable and, increasingly, non-communicable diseases might dominate allocation of health resources," Sullivan wrote, noting that just three low-income and lower-middle income countries contributed data to the study: Kenya, Paraguay, and India.
Highest estimated numbers of deaths in the western Pacific and Southeast Asia reflect large populations and are both thought to play key roles in global flu virus circulation, she said.
Though the study authors recommend wider use of flu vaccination to help drive down the flu mortality burden, Sullivan pointed out that current vaccine are imperfect and expected benefits might be overstated, especially for older people.
More research is needed to tease out the mortality contributions of different seasonal flu strains, and modeling methods designed to gauge the death-reduction benefits of flu vaccination programs need to factor in differences in flu vaccine effectiveness by subtype, lineage, age, and earlier vaccination history, she added.
Though the new report sheds light on seasonal flu mortality burden and the need for greater prevention, Sullivan wrote, "What is less clear is whether our current prevention strategies are adequate to reduce this burden."
Dec 13 Lancet study
Dec 13 Lancet commentary
Dec 13 CDC press release