Alternating patterns of seasonal influenza activity in the WHO European Region following the 2009 pandemic, 2010-2018

Abstract

Background: Influenza virus infections are common and lead to substantial morbidity and mortality worldwide. We characterized the first eight influenza epidemics since the 2009 influenza pandemic by describing the distribution of viruses and epidemics temporally and geographically across the WHO European Region.

Methods: We retrospectively analyzed laboratory-confirmed influenza detections in ambulatory patients from sentinel sites. Data were aggregated by reporting entity and season (weeks 40-20) for 2010-2011 to 2017-2018. We explored geographical spread using correlation coefficients.

Results: There was variation in the regional influenza epidemics during the study period. Influenza A virus subtypes alternated in dominance, except for 2013-2014 during which both cocirculated, and only one season (2017-2018) was B virus dominant. The median start week for epidemics in the Region was week 50, the time to the peak ranged between four and 13 weeks, and the duration of the epidemic ranged between 19 and 25 weeks. There was evidence of a west-to-east spread across the Region during epidemics in 2010-2011 (r = .365; P = .019), 2012-2013 (r = .484; P = .001), 2014-2015 (r = .423; P = .006), and 2017-2018 (r = .566; P < .001) seasons. Variation in virus distribution and timing existed within reporting entities across seasons and across reporting entities for a given season.

Conclusions: Aggregated influenza detection data from sentinel surveillance sites by season between 2010 and 2018 have been presented for the European Region for the first time. Substantial diversity exists between influenza epidemics. These data can inform prevention and control efforts at national, sub-national, and international levels. Aggregated, regional surveillance data from early affected reporting entities may provide an early warning function and be helpful for early season forecasting efforts.

Keywords: Central Asia; Europe; influenza; surveillance.

Figure 1

Map of member states of the WHO European Region. AL: Albania, AD: Andorra, AM: Armenia, AT: Austria, AZ: Azerbaijan, BY: Belarus, BE: Belgium, BA: Bosnia and Herzegovina, BG: Bulgaria, HR: Croatia, CY: Cyprus, CZ: Czech Republic, DK: Denmark, EE: Estonia, FI: Finland, FR: France, GE: Georgia, DE: Germany, GR: Greece, HU: Hungary, IS: Iceland, IE: Ireland, IL: Israel, IT: Italy, KZ: Kazakhstan, KG: Kyrgyzstan, LV: Latvia, LT: Lithuania, LU: Luxembourg, MT: Malta, MC: Monaco, ME: Montenegro, NL: Netherlands, NO: Norway, PL: Poland, PT: Portugal, MD: Republic of Moldova, RO: Romania, RU: Russian Federation, SM: San Marino, RS: Serbia, SK: Slovakia, SI: Slovenia, ES: Spain, SE: Sweden, CH: Switzerland, TJ: Tajikistan, MK: North Macedonia, TR: Turkey, TM: Turkmenistan, UA: Ukraine, GB: United Kingdom of Great Britain and Northern Ireland, and UZ: Uzbekistan. Note: The boundaries and names shown and the designations used on this map do not imply the expression of any opinion whatsoever on the part of the World Health Organization concerning the legal status of any country, territory or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted lines on maps represent approximate borderlines for which there may not yet be full agreement

Figure 2

Number of influenza viruses by type and subtype and percentage of specimens positive for influenza virus detected from sentinel outpatient surveillance in the WHO European Region by season, 2010‐2018. 10% and 40% positivity thresholds displayed

Figure 3

Total number of detections reported from sentinel outpatient surveillance in the WHO European Region by reporting entity and season, 2010‐2018. Data for Kosovo (In accordance with Security Council resolution 1244 (1999)). have not been presented

Figure 4

Distribution of influenza virus type, influenza A subtype and influenza B lineage from sentinel outpatient surveillance in the WHO European Region by season, 2010‐2018

Figure 5

Median and range of start weeks, peak weeks and durations for reporting entity‐level influenza epidemics in the WHO European Region by season, 2010 to 2018. Based on data from reporting entity sentinel surveillance outpatient clinics. The dots represent the median value for a season and the ends of the lines indicate the minimum and maximum values. The division between calendar years is indicated by the dashed vertical line, where appropriate. ISO week 53 in 2015 is not represented in the median start and peak plots but is included in measures of duration