Skip directly to site content Skip directly to page options Skip directly to A-Z link Skip directly to A-Z link Skip directly to A-Z link
Volume 24, Number 5—May 2018

Heterogeneous and Dynamic Prevalence of Asymptomatic Influenza Virus Infections

Cite This Article

To the Editor: We read with interest the article by Furuya-Kanamori et al. on the proportion of influenza virus infections that are asymptomatic or subclinical (1), and we are troubled by a series of fundamental flaws and errors. We were concerned that the authors presented pooled estimates of the asymptomatic fraction, given the massive heterogeneity in estimates (Ι2 values of 97%–98% in Technical Appendix Table 1). It is not considered good practice to present pooled estimates in instances of massive heterogeneity (2). We were very surprised that the authors included volunteer challenge studies because it is well known that the severity of these infections can be modulated by the route of administration and possibly the infectious dose. We also were surprised that human infections with avian influenza viruses were included because the epidemiology of these infections differs markedly from that of human influenza viruses. These studies were mistakenly labeled as studies of pandemic influenza in Technical Appendix Table 1. When reviewing serologic studies, the authors did not define a specific antibody titer threshold but relied on the choices made in individual studies; studies that inferred influenza virus infections based on low postepidemic hemagglutination-inhibition titers, such as 10 or 20, may lack specificity because some persons could have preexisting antibodies (3). Measurement error can also be a concern. The authors probably should have excluded such studies.

In another systematic review of the asymptomatic fraction of influenza virus infections (4), we found that study designs could explain a great deal of heterogeneity in the asymptomatic fraction in studies such as outbreak investigations that used molecular testing to confirm influenza virus infections rather than serologic studies that used antibody titer measurements to indicate infections. Asymptomatic fractions were higher in general, and much more heterogeneous, in studies that followed the latter approach.

Dr. Leung is a postdoctoral fellow at the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, The University of Hong Kong. Her research interests include the epidemiology and immunology of respiratory infections, including influenza.

Dr. Cowling is professor and division head of the Division of Epidemiology and Biostatistics at the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, The University of Hong Kong. His primary research interests are infectious disease epidemiology, particularly respiratory infections, including influenza.



The authors received financial support from the Harvard Center for Communicable Disease Dynamics from the National Institute of General Medical Sciences (grant no. U54 GM088558), the Area of Excellence Scheme of the University Grants Committee of Hong Kong (grant no. AoE/M-12/06), and a commissioned grant from the Health and Medical Research Fund from the Government of the Hong Kong Special Administrative Region.


Nancy H.L. Leung and Benjamin J. CowlingComments to Author 
Author affiliations: The University of Hong Kong, Hong Kong, China



  1. Furuya-Kanamori  L, Cox  M, Milinovich  GJ, Magalhaes  RJ, Mackay  IM, Yakob  L. Heterogeneous and dynamic prevalence of asymptomatic influenza virus infections. Emerg Infect Dis. 2016;22:10526. DOIPubMedGoogle Scholar
  2. Greenland  S, O’Rourke  K. Meta-analysis. In: Rothman KJ, Greenland S, Lash TL, editors. Modern epidemiology. 3rd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2008. p. 652.
  3. Broberg  E, Nicoll  A, Amato-Gauci  A. Seroprevalence to influenza A(H1N1) 2009 virus—where are we? Clin Vaccine Immunol. 2011;18:120512. DOIPubMedGoogle Scholar
  4. Leung  NH, Xu  C, Ip  DK, Cowling  BJ. The fraction of influenza virus infections that are asymptomatic: a systematic review and meta-analysis. Epidemiology. 2015;26:86272. DOIPubMedGoogle Scholar


Cite This Article

DOI: 10.3201/eid2405.160782

Original Publication Date: March 28, 2018

Related Links


Table of Contents – Volume 24, Number 5—May 2018

EID Search Options
presentation_01 Advanced Article Search – Search articles by author and/or keyword.
presentation_01 Articles by Country Search – Search articles by the topic country.
presentation_01 Article Type Search – Search articles by article type and issue.



Please use the form below to submit correspondence to the authors or contact them at the following address:

Benjamin J. Cowling, WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, The University of Hong Kong, 21 Sassoon Rd, Pokfulam, Hong Kong

Send To

10000 character(s) remaining.


Page created: April 17, 2018
Page updated: April 17, 2018
Page reviewed: April 17, 2018
The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.