Monitoring of influenza-like illness
As of the 2021/22 season, the sentinel surveillance system has been extended, allowing for the recruitment of more participating general practitioners (GPs). Furthermore, a larger number of samples per GP are collected and analysed than previously. The system has also been extended to cover the full calendar year, see EPI-NEWS no. 14/2022.
In the period from week 40/2022 to week 20/2023, an average of 139 GPs reported weekly to the sentinel surveillance system. In the autumn/winter, the share of patients who contacted their general practititoner (GP) with influenza-like symptoms (ILS) reached a moderate level. However, there were weeks where the number of patients surged to a high level. The surveillance also showed that patients contacted their GP with influenza-like symptoms in three waves occurring in the period from week 50 to week 12, see Figure 1. It should be noted that the consultation percentage is not specific for influenza but also reflects patients with COVID-19 and other respiratory infections who contact their GP with ILS . The activity recorded by the emergency call service physicians remained low throughout the season.
Among the approximately 6,500 influmeter participants, around 4,300 individuals reported weekly that they had experienced influenza-like symptoms. Throughout the season, the average share of Influmeter participants with influenza-like symptoms was 2.7% (1.5-4.5%), Figure 2. In this season, the reported ILS has remained at the same relatively low level as was recorded for the two preceding seasons. In contrast, in the pre-COVID-19 seasons (2017-2020), a larger share of patients reported ILS, ranging from moderate to high levels. This is to be expected as the current and previous season have mainly affected the adolescent part of the population, whereas Influmeter participants belong to the adult part of the population.
Laboratory-confirmed influenza cases
The 2022/23 influenza season was prolonged. It started early, in December 2022 and continued until mid-April 2023. From week 40/2022 to week 20/2023, the number of persons detected with influenza A (10,162) and influenza B (10,898) was similar and the cases were observed to occur in three waves. In the first wave, leading up to Christmas, influenza A was dominant. This was followed by a decline in the number of people who had influenza detected in January, a second wave was observed in February where influenza B was dominant. Finally, a smaller wave followed in mid-March when the numbers of influenza A and B cases were nearly identical (Figures 3 and 4). Since week 40, the testing activity has been extensive, though slightly lower than in the preceding season but higher than before the COVID-19 pandemic. Despite the high level of testing activity, the positive percentage remained at the same level as in the season leading up to the COVID-19 pandemic (2019/20).
Among the 21,060 confirmed influenza A and B cases, a total of 4,248 (20%) were admitted to hospital, among whom 356 (8%) received intensive care. Among the admitted patients, 232 died within 30 days after a positive influenza test, Table 1. Compared with the 2021/22 season, this corresponds to a 27% increase in the number of laboratory-confirmed influenza cases, whereas the number of persons who were admitted to the hospital was only 8% higher and the number of influenza-related deaths remained at the same level as reported in the 2021/22 season. Even though more people had influenza A than B detected, this did not lead to a corresponding increase in the number of hospital admissions or influenza-related deaths. This may probably be explained by the fact that the influenza subtypes in circulation primarily caused serious disease in the younger part of the population.
As the season was prolonged, the healthcare burden was smoother than in the 2021/22 season, which was very short and intense.
The number of influenza-related deaths was higher for influenza A (6.4%) than for influenza B (2.8%), Table 1. This may be attributed to influenza B mainly being detected in children and younger adults during this season. The need for intensive care among admitted patients was comparable for influenza A (8.2%) and influenza B (9.2%).
In the course of the season, the National Danish WHO Influenza Centre at SSI received a total of 9,140 samples for testing and characterisation of influenza viruses circulating during the season.
Among the typed samples, 2,190 were influenza A virus; among these, the A(H1N1)pdm09 subtype comprised 68% and the A(H3N2) subtype 32% (Figure 5). In all, 2,156 of the typed samples were influenza B virus, among which the lineage was determined in 1,298 cases, all of which belonged to the Victoria line (Figure 5). Influenza B/Victoria was dominant in the age groups 7-49 years, whereas B/Victoria was detected in only few persons aged 50 years or more (Figure 6). In the 50+ year age group, A(H1N1) dominated, followed by A(H3N2). An equal distribution was seen of B/Victoria and A(H1N1) in the 0-6-year age group.
Other respiratory viruses
SSI performed PCR-based testing of 4,765 samples submitted by GPs for more than 20 known respiratory viruses . Among these samples, 3,308 samples tested positive for one or more respiratory viruses. Figure 7 presents the development in detected respiratory viruses from week 40 2022 to week 20 2023 for all of Denmark. Apart from influenza, several different respiratory viruses were in circulation in the course of the season. In particular, respiratory syncytial virus (RSV) occurrence was atypical for the 2021/22 season and again for the 2022/23 season. The RSV season is described in more detail in the next section. Rhinovirus, is another virus detected in the submitted samples, that caused a large wave of cases in the autumn of 2022. SARS-CoV-2 was present in all weeks of the 2022/23 season.
Respiratory syncytial virus (RSV)
Prior to COVID-19, a typical RSV season would start around week 49, peak in the late winter months and typically end around week 18. During the social lockdown in 2020/21, only sporadic cases of RSV were recorded. As society was gradually reopened in March 2021, RSV returned, displaying a very atypical course. The virus appeared in the early summer of 2021 and peaked during the summer months, see EPI-NEWS 35 - 2021. In the completed 2022/23 season, this pattern was repeated. However, the 2022/23 season did not start until late in the summer of 2022 (Figure 8).
A remarkable characteristic of the 2022/23 season was the prolonged period during which RSV was observed; from late summer to late January, causing a relatively high number of admissions and, as mentioned, extending across a longer period than was the case in previous seasons (Figure 9). One explanation for this may be the age distribution. In Denmark, RSV infection among children typically occurs at a different time than in adults/elderly people (65+-year-olds). The 2022/23 season was characterised by two overlapping RSV waves: the first wave affected young children aged 0-2 years, and peaked around week 44, and the second wave affected people aged 65 years or more, and peaked in week 51. This increase in the number of confirmed cases in persons aged 65 years and above also caused more admissions among persons over 65 years and more RSV-related deaths among persons over 65 years. During the 2021 summer epidemic, the number of RSV-related deaths was 99, compared with 287 deaths in the 2022/23 season. In the two seasons leading up to COVID-19, the number of deaths with RSV was approximately 50-60 people.
Two subtypes of RSV exist: RSV-A and RSV-B. During the 2021 summer epidemic, RSV-A was dominant early in the season, followed by a smaller overlapping wave of RSV-B. RSB-B was detected sporadically throughout winter and became dominant in the 2022/23 season. Only sporadic cases of RSV-A were detected. Genetic characterisation of the circulating RSV of type A and B showed similarity with RSV found in other countries in the same time period/season.
Vaccine coverage and vaccine effectiveness
In this season, seasonal influenza vaccination was offered to risk groups. Furthermore, for the third year, vaccination was offered to healthcare and nursing staff and for the second year to children aged 2-6 years. Children were offered vaccination with a nasal spray with live attenuated influenza virus, whereas the remaining target groups were offered vaccination with inactivated influenza vaccine.
For persons aged 65 years or above, a high coverage exceeding 80% was achieved, whereas the coverage among the remaining target groups was somewhat lower. For example, healthcare and nursing staff had a coverage of 21%, pregnant women in their 2nd and 3rd trimester had a coverage of 27% and children aged 2-6 years had a coverage of 22% (Table 2). It should be noted that the coverage rates of several target groups that cannot be stated by age were based on registry data extractions, and therefore the estimated vaccine coverage is associated with some uncertainty.
The vaccine effectiveness (VE) to influenza A and B was high for children aged 2-6 years, reaching 78% (95% confidence interval (CI): 71-84%) and 95% (95% CI: 92-98%), respectively. In the group of 18-64-year-olds, VE was moderate for influenza A, 41% (95% CI: 36-46%), and high for influenza B, 70% (95% CI: 66-73%) In the group of persons aged 65 years or above, VE to influenza
A was low, 26% (95% CI: 19-33%), whereas it was high for influenza B, 64% (95% CI: 53-73%). As in previous years, the influenza vaccine had a good effect against influenza B. Similarly, the live-attenuated vaccine given to children aged 2-6 years has proven to be highly effective. In line with previous seasons, the effect of the vaccines against influenza A among adults and in the older age groups is more moderate.
Transmission of influenza from animals to humans and outbreaks with highly pathogenic avian influenza in birds
No cases were detected of influenza virus transmission from animals to humans in Denmark in 2022. However, an increasing number of highly pathogenic influenza viruses have been detected in various bird species.
Influenza in pigs
In 2022, the monitoring of influenza virus in Danish pigs was based on samples from 450 diagnostic submissions from 439 pig farms. The samples were submitted for testing for influenza virus at the Danish veterinary laboratories, the SSI and the Veterinary Laboratory in Kjellerup. Typically, 3-5 samples were submitted from each farm. The submissions were received from all parts of Denmark, and influenza virus was detected in all parts of the year, even though the summer semester recorded fewer submissions and a lower share of positive submissions. Influenza A virus was detected in 68% of the submissions. The most common subtypes were H1avN2sw, H1pdm09N1av and H1N1pdm09. For more information, please see the monitoring page of DK-VET.
Influenza in birds and mammals
Outbreaks with clade 22.214.171.124b H5 highly pathogenic avian influenza (HPAI) virus continued in Europe in 2022, with extraordinarily many findings in both wild birds and poultry. In Denmark, outbreaks were observed in nine poultry farms in 2022 prompted by detection of clinical signs raising suspicion of HPAI. In the same period, HPAI virus was detected in more than 200 dead wild birds - primarily geese, swans, buzzards and seagulls - found in all parts of Denmark and reported to the authorities (https://ai.fvst.dk/). Furthermore, HPAI virus was detected in a limited number of cases in apparently healthy hunted wild ducks during the autumn of 2022. The viruses found in the affected farms were the same as those detected in wild birds. This is part of the reason why it is assumed that the virus was introduced at the farms through direct or indirect contact with wild birds. Read more at the website of the Danish Veterinary and Food Administration.
Worldwide, there have been several cases of highly pathogenic H5 viruses transmitted from birds to mammals. The infected mammals have either been found dead or ill, presenting with neurological symptoms. The affected species are primarily foxes and sea mammals, but cases have also been observed in a wide range of wild predators and a limited number of cases in dogs and cats. 2022 saw the first-ever detection of HPAI virus in Danish foxes.
In Denmark, avian influenza virus has never been detected in humans, but serious and non-serious cases have been recorded in other countries. In the overwhelming majority of cases, close contact with infected birds or contaminated surroundings was documented.
This annual report is also discussed in EPI-NEWS no. 25/2023.