Last update:
19-Jan-2023

OBJECTIVE To estimate the R₀ and R_t of influenza A (H1N1) 2009 pandemic in Greece and to compare these estimates with those reported in the literature for the influenza A (H1N1) 2009 pandemic in other countries, and with those of past influenza pandemics, seasonal influenza and the COVID-19 pandemic.

METHOD We used data on the number of laboratory-confirmed influenza A (H1N1) 2009 cases per week for the period from mid-September 2009 to February 2010 in Greece that were collected by the Hellenic Centre for Diseases Control and Prevention. The daily number of cases was obtained, using linear and cubic spline interpolation to estimate the R_t. R₀ was estimated applying the maximum likelihood method on the daily number of cases obtained from cubic spline interpolation. Finally, sensitivity analysis was performed to assess the robustness of the R₀ estimate.

RESULTS By the end of October 2009, the number of laboratory-confirmed cases of influenza A (H1N1) 2009 increased exponentially, reaching a peak at the end of November. The estimated basic reproduction number R₀ was 1.35 (95% confidence interval: 1.16–1.57). R_t was close to 1 in mid-October 2009; it increased subsequently and reached close to 1.4 in early November, then in December 2009 it declined below 1 and remained at low levels until the end of February 2010.

CONCLUSIONS The estimation of the basic reproduction number of pandemic influenza A (H1N1) 2009 in Greece is in line with estimates provided by other countries, and places R₀ at lower levels compared to R₀ estimates for both previous influenza pandemics and the COVID-19 pandemic. In addition, the estimated 2009 R₀ is similar to the higher estimates for R₀ of seasonal influenza.

Key words: Basic reproduction number, COVID-19 pandemic, Effective reproduction number, Influenza A (H1N1) 2009, Influenza pandemics.