Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature

Abstract

Background: The potential impact of an influenza pandemic can be assessed by calculating a set of transmissibility parameters, the most important being the reproduction number (R), which is defined as the average number of secondary cases generated per typical infectious case.

Methods: We conducted a systematic review to summarize published estimates of R for pandemic or seasonal influenza and for novel influenza viruses (e.g. H5N1). We retained and summarized papers that estimated R for pandemic or seasonal influenza or for human infections with novel influenza viruses.

Results: The search yielded 567 papers. Ninety-one papers were retained, and an additional twenty papers were identified from the references of the retained papers. Twenty-four studies reported 51 R values for the 1918 pandemic. The median R value for 1918 was 1.80 (interquartile range [IQR]: 1.47-2.27). Six studies reported seven 1957 pandemic R values. The median R value for 1957 was 1.65 (IQR: 1.53-1.70). Four studies reported seven 1968 pandemic R values. The median R value for 1968 was 1.80 (IQR: 1.56-1.85). Fifty-seven studies reported 78 2009 pandemic R values. The median R value for 2009 was 1.46 (IQR: 1.30-1.70) and was similar across the two waves of illness: 1.46 for the first wave and 1.48 for the second wave. Twenty-four studies reported 47 seasonal epidemic R values. The median R value for seasonal influenza was 1.28 (IQR: 1.19-1.37). Four studies reported six novel influenza R values. Four out of six R values were <1.

Conclusions: These R values represent the difference between epidemics that are controllable and cause moderate illness and those causing a significant number of illnesses and requiring intensive mitigation strategies to control. Continued monitoring of R during seasonal and novel influenza outbreaks is needed to document its variation before the next pandemic.

Figure 1

PRISMA flowchart of the article selection for the reproductive number and influenza literature review.

Figure 2

Estimates of the reproduction number for the 1918 influenza A/H1N1 pandemic according to location, wave of illness, setting, and the serial interval or generation time assumed in the estimation method. For individual studies, the single estimate or median of multiple estimates is shown as a circle for basic reproduction numbers or a square for effective reproduction numbers, and the range or confidence interval is denoted by brackets. Estimates of the reproduction number are color coded based on the generation time or serial interval used in calculations: red (<3 days), blue (≥3 days), or black (not reported or not used).

Figure 3

Estimates of the reproduction number for the 1957 influenza A/H2N2 and the 1968 influenza A/H3N2 pandemics according to location, wave of illness, setting, and the serial interval or generation time assumed in the estimation method. For individual studies, the single estimate or median of multiple estimates is shown as a circle for basic reproduction numbers or a square for effective reproduction numbers, and the range or confidence interval is denoted by brackets. Estimates of the reproduction number are color coded based on the generation time or serial interval used in calculations: red (<3 days), blue (≥3 days), or black (not reported or not used).

Figure 4

Estimates of the reproduction number for the 2009 Influenza A/H1N1 pandemic according to location, wave of illness, setting, and the serial interval or generation time assumed in the estimation method. For individual studies, the single estimate or median of multiple estimates is shown as a circle for basic reproduction numbers or a square for effective reproduction numbers, and the range or confidence interval is denoted by brackets. Estimates of the reproduction number are color coded based on the generation time or serial interval used in calculations: red (<3 days), blue (≥3 days), or black (not reported or not used).

Figure 5

Estimates of the reproduction number in the community for seasonal influenza epidemics according to location, wave of illness, and the serial interval or generation time assumed in the estimation method. For individual studies, the single estimate or median of multiple estimates is shown as a circle for basic reproduction numbers or a square for effective reproduction numbers, and the range or confidence interval is denoted by brackets. Estimates of the reproduction number are color coded based on the generation time or serial interval used in calculations: red (<3 days), blue (≥3 days), or black (not reported or not used).