Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2013 Nov;24(6):830-41.
doi: 10.1097/EDE.0b013e3182a67448.

Case fatality risk of influenza A (H1N1pdm09): a systematic review

Jessica Y Wong  1 , Heath Kelly, Dennis K M Ip, Joseph T Wu, Gabriel M Leung, Benjamin J Cowling
Affiliations

Case fatality risk of influenza A (H1N1pdm09): a systematic review

Jessica Y Wong et al. Epidemiology. 2013 Nov.

Abstract

Background: During the 2009 influenza pandemic, uncertainty surrounding the seriousness of human infections with the H1N1pdm09 virus hindered appropriate public health response. One measure of seriousness is the case fatality risk, defined as the probability of mortality among people classified as cases.

Methods: We conducted a systematic review to summarize published estimates of the case fatality risk of the pandemic influenza H1N1pdm09 virus. Only studies that reported population-based estimates were included.

Results: We included 77 estimates of the case fatality risk from 50 published studies, about one-third of which were published within the first 9 months of the pandemic. We identified very substantial heterogeneity in published estimates, ranging from less than 1 to more than 10,000 deaths per 100,000 cases or infections. The choice of case definition in the denominator accounted for substantial heterogeneity, with the higher estimates based on laboratory-confirmed cases (point estimates = 0-13,500 per 100,000 cases) compared with symptomatic cases (point estimates = 0-1,200 per 100,000 cases) or infections (point estimates = 1-10 per 100,000 infections). Risk based on symptomatic cases increased substantially with age.

Conclusions: Our review highlights the difficulty in estimating the seriousness of infection with a novel influenza virus using the case fatality risk. In addition, substantial variability in age-specific estimates complicates the interpretation of the overall case fatality risk and comparisons among populations. A consensus is needed on how to define and measure the seriousness of infection before the next pandemic.

PubMed Disclaimer

Conflict of interest statement

CONFLICTS OF INTEREST

The authors report no other potential conflicts of interest.

Figures

Figure 1
Figure 1
Flow diagram of study selection.
Figure 2
Figure 2
Study dates (solid horizontal lines), eventual publication dates of the studies included in the review (points) compared with the histogram of confirmed H1N1pdm09 deaths reported to the World Health Organization (underlying histogram). See Table for details of each study.
Figure 3
Figure 3
Estimated risk of death by eventual publication dates of the studies included in the review (points with 95% CI) compared with the histogram of confirmed H1N1pdm09 deaths reported to the World Health Organization (underlying histogram).
Figure 4
Figure 4
Age-specific estimated risk of death.
See this image and copyright information in PMC

Comment in

References

    1. Van Kerkhove MD, Asikainen T, Becker NG, et al. Studies needed to address public health challenges of the 2009 H1N1 influenza pandemic: insights from modeling. PLoS Med. 2010;7(6):e1000275. doi: 10.1371/journal.pmed.1000275. - DOI - PMC - PubMed
    1. Lipsitch M, Hayden FG, Cowling BJ, et al. How to maintain surveillance for novel influenza A H1N1 when there are too many cases to count. Lancet. 2009;374(9696):1209–11. doi: 10.1016/S0140-6736(09)61377-5. - DOI - PubMed
    1. Boelle PY, Ansart S, Cori A, et al. Transmission parameters of the A/H1N1 (2009) influenza virus pandemic: a review. Influenza Other Respi Viruses. 2011;5(5):306–16. doi: 10.1111/j.1750-2659.2011.00234.x. - DOI - PMC - PubMed
    1. World Health Organization. Implementation of the International Health Regulations (2005) Report of the review committee on the functioning of the International Health Regulations (2005) in relation to pandemic (H1N1) 2009. 2011
    1. Nishiura H. Case fatality ratio of pandemic influenza. Lancet Infect Dis. 2010;10(7):443–4. doi: 10.1016/S1473-3099(10)70120-1. - DOI - PMC - PubMed

Publication types