Epidemiology of Influenza During 2010/2011 in Lithuania
Published: 13.08.12 Updated: 17.08.2012 13:38:26
S. Čaplinskas 1,2, G. Šeškaitė 1, D. Razmuvienė 1
1 Centre for Communicable Diseases and AIDS, Vilnius, Lithuania
2 Mykolas Romeris University, Vilnius, Lithuania
Citation: Čaplinskas S, Šeškaitė G, Razmuvienė D. Epidemiology of Influenza During 2010/2011 in Lithuania. EpiNorth 2011; 12:72-5.
Influenza and acute respiratory infections cause serious health problems in addition to social and economic challenges. In this article the 2010/2011 influenza season is reviewed and compared with the pandemic 2009/2010 influenza season using data from the Centre for Communicable Diseases and AIDS. The last influenza season was more or less similar to all influenza seasons before the pandemic season. However, there were almost as many deaths as during the pandemic season. None of the patients who had lethal outcomes were vaccinated. The biggest difference from the pandemic season was that the most common influenza virus detected in samples was A(H1)v during the 2010/2011 influenza season. The differences and similarities with the pandemic season and the unpredictability of every influenza season are discussed in this article.
Influenza and acute respiratory infections (ARI) are a global health care problem. These infections constitute a major part of all communicable diseases and have serious implications for health as well as economic and social consequences [3, 4]. Influenza and ARI are more common in the Northern countries and especially during the winter season. Approximately 200,000 ARI cases and 50,000 influenza cases are registered in Lithuania every influenza season. Influenza and ARI can cause serious illness and may result in lethal outcomes.
Epidemiological surveillance over influenza and ARI in Lithuania during 2010/2011 season is examined in this study. It was difficult to compare this season with the 2009/2010 season because of the influenza pandemic that was characterized by earlier and higher morbidity than usual.
We used case definitions for influenza and ARI according to European Commission Decision of 28 April 2008 amending Decision 2002/253/EC that provides case definitions for reporting communicable diseases to the Community network under Decision No 2119/98/EC of the European Parliament and of the Council. Data from the Centre for Communicable Diseases and AIDS were used and were collected from weekly statistical influenza reports. All cases of clinically and laboratory-confirmed influenza and all clinically diagnosed ARI cases of the upper respiratory tract were registered for the entire country from week 40 of 2010 until week 20 of 2011 and from week 40 of 2009 until week 20 of 2010. Patients with lower respiratory tract ARI or SARI (severe acute respiratory infection) were not included in the analysis as this information is not reported.
We compared 2010/2011 seasonal influenza season data with data of the pandemic 2009/2010 influenza season. We also accessed medical records from all hospitalized cases in Lithuania and information regarding lethal outcomes from influenza and ARI during both seasons. Hospitalization data included children 0 -17 years of age and adults from 18 years and older. These data also include all intensive care cases and all pregnant women who were hospitalized for influenza or ARI.
Analysis of epidemiological data indicates that during the 2010/2011 influenza season 609,183 people were infected with influenza or ARI viruses; 7% (41,849) of the total were influenza cases. During the 2009/2010 season this number was larger and reached 13% (58,264) of all influenza and ARI cases (456,760) (Figure 1). During this influenza season the incidence peak was observed from January until March as is typical for a normal influenza season. During the pandemic influenza season 2009/2010 the incidence peak was registered from November until December, which was unusually early and higher than observed with seasonal influenza.
The incidence of influenza and ARI in Lithuania during the last two influenza seasons is shown in Figure 1.
Fig. 1. Incidence of influenza and ARI in 2009/2010 and 2010/2011 influenza seasons in Lithuania
Epidemics were registered in 22 districts (37% of all the districts) during the last influenza season. In one district an epidemic was registered twice. The first local epidemic was registered during the third week of 2011 (January 17-23) and included only half as many districts as the pandemic season. Epidemics were registered in 56 districts (93% of the districts) during the 2009/2010 influenza pandemic.
The number of hospitalized patients indicates influenza and ARI incidence, how often infected patients go to medical institutions and the need for health resources. A total of 3,326 patients were hospitalized during the last influenza season. A majority, 67% (2,238), were 17 years old or younger. Four percent (146 persons) of the hospitalized patients were treated in intensive care units. The number of intensive care patients was significantly lower than during the pandemic influenza season (208 persons). Pregnant women accounted for 1% (34 persons) of the hospitalized patients, approximately 50% less than during the pandemic season (57 persons). The highest number of hospitalized patients was registered from 4 to 7 week of 2011. The rate of hospitalizations is correlated to incidence.
From the beginning of the 2010/2011 influenza season 1,695 samples from patients with influenza and ARI were received at the Lithuanian National Laboratory as compared to 2,239 samples during the 2009/2010 influenza season. Twenty one percent (349) of the samples were A(H1)v-positive. Other influenza viruses were less frequent and included 8% (132) B type influenza virus, 5% (90) A(H1N1) influenza virus and 3% (44) A type influenza virus. Two viruses in one specimen were found in 10 samples (2 samples with A and B influenza viruses and 8 samples with A(H1)v and B influenza viruses). The most common virus (33%) detected during the pandemic season was A(H1N1).
Influenza or ARI complications were responsible for the death of 21 patients as confirmed by the Lithuanian National Laboratory. This number is slightly lower than during the pandemic influenza season when 23 patients had lethal outcomes. Data indicates that almost all patients who had lethal outcomes during the last influenza season were 10 or more years older than during the pandemic season (Figure 2).
Fig. 2. Case fatality rate from influenza and ARI by age groups in 2009/2010 and 2010/2011 influenza seasons in Lithuania
None of the patients who had lethal outcomes were vaccinated. The number of vaccinated persons during the last influenza season in Lithuania was 99,490, approximately 3% of the population. A little more than half (56%) of the vaccinated persons were 65 years or older. During the pandemic season 4% of the population were vaccinated. The highest vaccination coverage during the 2010/2011 influenza season was in October (51.5%) and the lowest - in February (0.6%).
In Lithuania, the 2010/2011 influenza season was similar to a usual seasonal influenza. During the pandemic season, incidence of influenza and ARI was at least twice as high as reported for seasonal epidemics. Lethal outcomes were similar for the two recent influenza seasons.
The incidence peak occurred from January until March. The highest hospitalization rates were registered among people younger than 17 years. This may be due to the fact that children had a more severe clinical form of the disease or sought medical attention more often than patients in other age groups. The share of lethal outcomes among people over the age of 60 was higher than during the pandemic season. None of the patients who had lethal outcomes were vaccinated thus indicating the importance of vaccination. Only 3% of the population was vaccinated during this season. One possible explanation could be that influenza vaccine in Lithuania is free of charge only for risks groups.
The most common laboratory-confirmed virus during this season was A(H1)v influenza. The pandemic influenza A(H1N1)2009 virus was confirmed in just a few percent of all samples.
Lithuania has learned an important lesson from the pandemic season and was much better prepared for the 2010/2011 influenza season. Like almost every European country, Lithuania has a preparedness plan for pandemic influenza. However, the influenza virus is insidious and, even though we have a much better understanding than before the pandemic of 2009/2010, we must remain vigilant.
1. Lithuanian statistics data. http://www.stat.gov.lt/lt/
2. Lithuanian Centre for Communicable Diseases and AIDS. Statistical weekly influenza reports. Available online: http://www.ulac.lt/index.php?pl=107&ppl=1#sergamumas
3. ECDC interim guidance: Interim ECDC public health guidance on case and contact management for the new influenza A(H1N1) virus infection.-Version 3.-2009. Available online: http://ecdc.europa.eu/en/publications/Publications/0905_GUI_Influenza_AH1N1_Public_Health_Guidance_on_Case_and_Contact_Management.pdf ;
4. Epidemic alert and response: WHO checklist for influenza pandemic preparedness planning. WHO/CDS/CSR/GIP/2005.4.- Switzerland, 2005.