Present Status of Knowledge Concerning Influenza

Nonannual seasonality of influenza-like illness in a tropical urban setting

BACKGROUND

In temperate and subtropical climates, respiratory diseases exhibit seasonal peaks in winter. In the tropics, with no winter, peak timings are irregular.

METHODS

To obtain a detailed picture of influenza-like illness (ILI) patterns in the tropics, we established an mHealth study in community clinics in Ho Chi Minh City (HCMC). During 2009-2015, clinics reported daily case numbers via SMS, with a subset performing molecular diagnostics for influenza virus. This real-time epidemiology network absorbs 6000 ILI reports annually, one or two orders of magnitude more than typical surveillance systems. A real-time online ILI indicator was developed to inform clinicians of the daily ILI activity in HCMC.

RESULTS

From August 2009 to December 2015, 63 clinics were enrolled and 36 920 SMS reports were received, covering approximately 1.7M outpatient visits. Approximately 10.6% of outpatients met the ILI case definition. ILI activity in HCMC exhibited strong nonannual dynamics with a dominant periodicity of 206 days. This was confirmed by time series decomposition, stepwise regression, and a forecasting exercise showing that median forecasting errors are 30%-40% lower when using a 206-day cycle. In ILI patients from whom nasopharyngeal swabs were taken, 31.2% were positive for influenza. There was no correlation between the ILI time series and the time series of influenza, influenza A, or influenza B (all P > 0.15).

CONCLUSION

This suggests, for the first time, that a nonannual cycle may be an essential driver of respiratory disease dynamics in the tropics. An immunological interference hypothesis is discussed as a potential underlying mechanism.

Statistical identifiability and sample size calculations for serial seroepidemiology

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We investigate whether disease dynamics can be inferred by repeated serum collections.

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Measuring antibody waning is critical for inference in serological time series.

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Collecting 200 samples every 2 months allows for inference of transmission parameters.

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Low-level seasonality is difficult to detect statistically.

Inference on disease dynamics is typically performed using case reporting time series of symptomatic disease. The inferred dynamics will vary depending on the reporting patterns and surveillance system for the disease in question, and the inference will miss mild or underreported epidemics. To eliminate the variation introduced by differing reporting patterns and to capture asymptomatic or subclinical infection, inferential methods can be applied to serological data sets instead of case reporting data. To reconstruct complete disease dynamics, one would need to collect a serological time series. In the statistical analysis presented here, we consider a particular kind of serological time series with repeated, periodic collections of population-representative serum. We refer to this study design as a serial seroepidemiology (SSE) design, and we base the analysis on our epidemiological knowledge of influenza. We consider a study duration of three to four years, during which a single antigenic type of influenza would be circulating, and we evaluate our ability to reconstruct disease dynamics based on serological data alone. We show that the processes of reinfection, antibody generation, and antibody waning confound each other and are not always statistically identifiable, especially when dynamics resemble a non-oscillating endemic equilibrium behavior. We introduce some constraints to partially resolve this confounding, and we show that transmission rates and basic reproduction numbers can be accurately estimated in SSE study designs. Seasonal forcing is more difficult to identify as serology-based studies only detect oscillations in antibody titers of recovered individuals, and these oscillations are typically weaker than those observed for infected individuals. To accurately estimate the magnitude and timing of seasonal forcing, serum samples should be collected every two months and 200 or more samples should be included in each collection; this sample size estimate is sensitive to the antibody waning rate and the assumed level of seasonal forcing.

NEUTRALIZING ANTIBODIES IN HUMAN SERUM AFTER INFLUENZA A : THE LACK OF STRAIN SPECIFICITY IN THE IMMUNOLOGICAL RESPONSE

The increased concentrations of neutralizing antibodies against influenza A virus in human serum which occur after influenza A do not differentiate between antigenically different strains of this virus or swine influenza virus but instead appear to possess equal reactivity against these agents. The decrease in antibody levels which occurs with time is also independent of the strain of virus used to measure it.

A COMPLEX VACCINE AGAINST INFLUENZA A VIRUS : QUANTITATIVE ANALYSIS OF THE ANTIBODY RESPONSE PRODUCED IN MAN

A quantitative study of the antigenicity of various vaccines containing influenza A virus has been made in human beings. A complex vaccine prepared from chick embryos inoculated with both influenza A virus and the X strain of canine distemper virus was found to be more effective than other vaccines in stimulating the production of neutralizing antibodies against the former virus. The increased antibody levels which resulted from the administration of this vaccine remained almost unaltered for at least 5 months.