This season, the flu started about four weeks earlier than in the previous years. By the beginning of February, CIPH has been receiving reports on the increased number of patients with flu from all over Croatia, influenza diagnostics laboratory received a large number of samples and the percentage of positive results among the analysed samples was continuously growing.
So far, through the normal reporting system based on the clinical picture (with or without laboratory confirmation), 45,852 flu cases were reported in Croatia. Most reports came from the City of Zagreb (7399) and from the Split-Dalmatia County (5,674).
The number of flu reports
In the last week, 4,716 flu cases were reported, which is a significantly smaller number of reports compared to the previous four weeks. In the previous week, the largest number of reports came from Zagreb (748), Zadar (611) and Varaždin County (408).
The number of influenza reports per weeks in this season and in the previous three seasons is shown in Figure 1.
Based on the above it can be concluded that the influenza passed its peak and is now in a downward trend, which is also expected to continue in the following weeks.
Mortality during the influenza season
To date, via individual reports of diseases/deaths from infectious diseases, Croatian Institute of Public Health has received thirteen official reports of influenza-related deaths (via aggregated weekly reports; 15 reports were received of influenza-related deaths, which means that two more official reports of influenza-related deaths are expected).
Influenza-related deaths, unfortunately, happen every season; Persons who die usually have weakened immune systems, develop complications (mainly pneumonia) or have chronic diseases which aggravate due to flu, especially among the elderly.
The actual number of deceased from influenza is never officially registered due to a number of reasons: e.g., sometimes influenza is not suspected as a possible cause of death, samples for testing are not timely taken or a deceased person is not tested for flu for some other reason. Therefore, in estimating the impact of influenza on human health and mortality, the term “excess mortality during the influenza season / winter” is used, determining the estimate of the number of patients who die, and whose death is associated with the flu, whereas in fact, for the majority of deaths it is never found out whether the flu contributed to death.
For a country with a population similar to Croatian, it is estimated that this excess mortality during the influenza season amounts to 125-550 people per year, depending on the intensity of the flu. This means that it is estimated that in every flu season, not just this year, in the weeks when influenza activity is increased, approximately 2-10% (depending on the season) more people die than they would if there was no flu.
Countries with IT infrastructure for timely monitoring of deaths are involved in a European project for monitoring excess mortality during winter (EuroMOMO), whose reports are publicly available at the web address: http://www.euromomo.eu/.
From the latest such report, Figure 2 shows the estimates of excess deaths this winter compared to previous ones. It is evident that in parallel with the increase in flu intensity during the first weeks of this year, excess mortality is also growing (red curve), reaching nearly 10% in the fourth week, and starting to slowly decline in the last week shown (fifth week of 2017). This surplus can largely be attributed to excess mortality in patients older than 65; The report available at the EuroMOMO project website shows more detailed data, presented by age groups.
Earlier beginning of the flu season has historically been associated with longer duration of the flu season and a higher total number of patients compared to later beginning of the flu season (e.g. in January or February). However, if the downward trend in the number of new cases continues, this season will not exceed the penultimate season regarding the number of diseased.
Every flu season, the biggest morbidity (incidence) is among school-age children, followed by the pre-school children, and hen among adults, while among the elderly there is usually the lowest incidence. However, elderly people have the highest risk of developing complications caused by influenza. In seasons when the prevalent subtype is A/H3N2, there is usually a slightly larger representation among elderly patients than in seasons when the prevalent subtype of influenza is A/H1N1.
This season, the tendency of increased share of the elderly in the seasons when the prevailing influenza virus is A/H3N2 in relation to the seasons when the prevailing virus is A/H1N1 is evident. While in the previous seasons the reported number of patients over the age of 65 was 5-7%, this season, so far, 10.3% of reported patients were older than 65.
Since the elderly are at a higher risk of developing complications due to influenza, the European Centre for Disease Prevention and Control published a risk assessment related to flu for this season, according to which one can expect an increased number of hospitalizations from influenza compared to the previous few seasons. Additional recommendation that they state, and that is supported by CIPH, is that when people with chronic diseases or people who are at an increased risk from complications get the flu, they should apply antiviral therapy with neuraminidase inhibitors as soon as possible, regardless of whether they are vaccinated against flu or not.
Estimations of the flu vaccine effectiveness for this season are being prepared and are expected to be within the average efficiency of the last few seasons in which the prevailing influenza virus was the subtype A/H3N2, regularly somewhat lower than in the seasons when the prevailing subtype A/H1N1 influenza virus.
To reduce the risk of infection in the following few weeks of expected flu season, it is recommended to avoid crowded enclosed spaces and to frequently wash hands. Patients are recommended to wear disposable surgical masks when going to the doctor and to properly use the disposable tissues (throw in the trash after each use), cover the mouth when coughing and sneezing and to frequently wash their hands.