Monitoring of Influenza and Influenza-Like Viruses in the 2022/2023 and 2023/2024 Epidemic Seasons Using the SENTINEL and NON-SENTINEL Surveillance Systems in Poland

Introduction

Influenza is a contagious respiratory infection, known since the 16th century, that spreads quickly during outbreaks. Influenza is caused by types A and B viruses, and occasional pandemics are triggered exclusively by influenza A viruses [1]. In the 20th century, there were 3 significant influenza pandemics in the world: in 1918–1919 Spanish flu, caused by the A/H1N1/subtype, the 1957–1958 Asian flu pandemic, caused by the A/H2N2/subtype, and the 1968–1969 Hong Kong pandemic caused by the A/H3N2/subtype [2]. The rapid spread of the disease is attributed to both the increased mobility of people and the growing volume of international travel, as well as the presence of animal reservoirs, which contribute to the emergence of new strains and subtypes of the influenza virus [3].

The World Health Organization (WHO) recognized the far-reaching consequences of the Spanish flu pandemic in the 20th century, not only in terms of health outcomes, estimated to have caused 50–100 million deaths, but also the severe economic impact. Therefore, global virological and epidemiological research program was established in the form of international influenza surveillance [4]. The WHO Global Influenza Surveillance Network has been continuously improved and in 2011 it was renamed the Global Influenza Surveillance and Response System (GISRS) [3].

Currently, the network consists of 7 International Reference Centres worldwide, along with 152 National Influenza Centres (NICs) in 130 countries [5]. In Poland, the NIC is located at the National Institute of Public Health and the National Institute of Hygiene – National Research Institute (NIPH, NIH-NRI). In the 2004/2005 epidemic season, after meeting WHO’s requirements, Poland established the SENTINEL Influenza Surveillance System. The system, which includes cooperation with 16 VSES, monitors circulating respiratory viruses [6]. The materials for the studies consist of throat and nasal swabs, as well as bronchial washings. A model for weekly epidemiological reporting was developed for collecting patient interviews, and training in molecular biology techniques of real-time RT-PCR and RT-PCR was provided. The VSES laboratories were equipped with appropriate equipment and mandated their active participation in SENTINEL supervision, as well as to support the development of this surveillance system at a provincial level, particularly by establishing cooperation with family medicine physicians, whose involvement was essential to its functioning. Clinical surveillance of influenza was conducted by the European Influenza Surveillance Network (EISN) and consisted of weekly reporting of confirmed and suspected influenza cases during the epidemic season by participating physicians [7].

Currently, due to the digitalization of data in Poland, family physicians no longer complete the MZ-55 Report on suspected cases of influenza and influenza-like viruses, which was previously submitted to the Epidemiology Department of the National Institute of Public Health, National Institute of Hygiene – National Research Institute. Instead, data are now transmitted directly to IT systems as part of medical events to the eHealth Center system. Reporting of this document has been suspended from July 1, 2023 [8]. The quality of virological and epidemiological surveillance for seasonal influenza and highly pathogenic influenza viruses depends on national government policies, economic conditions, and the public health and economic threats posed by this pathogen.

The aim of this study was to comparatively analyze circulating influenza viruses and influenza-like viruses during the 2022/2023 and 2023/2024 epidemic seasons, based on data from the SENTINEL and NON-SENTINEL surveillance systems in Poland.

Material and Methods

SURVEILLANCE SYSTEM:

Poland has been a part of the Global Influenza Surveillance and Response System (GISRS). Since the 2004/2005 influenza season, surveillance has been conducted through the SENTINEL system, which tracks both virological and epidemiological data. NIC acts as the coordinator of the system. Monitoring takes place throughout the entire influenza season, with the highest activity usually occurring from January to April, consistent with seasonal patterns in the northern hemisphere. Data are collected on a weekly basis and include:

The system includes data from both SENTINEL and NON-SENTINEL sources. The group of doctors participating in SENTINEL surveillance, who perform these activities on an honorary basis, should constitute 1-5% of practicing doctors in the country. They are responsible for collecting swabs and gathering information on the patient’s health status using paper-based reports. The collected swabs are sent to the VSES via the Provincial Sanitary and Epidemiological Station. Epidemiological data are collected across 4 age groups: 0–4, 5–14, 15–64, and over 65 years of age. Each VSES compiles a weekly report containing both virological and epidemiological data and submits it to the system. Only the NIC, as the coordinator of the system, has access to the entered data. Currently, data are aggregated and forwarded as weekly reports to the TESSy platform. NON-SENTINEL samples are collected from hospitals, public institutions, and private healthcare facilities that do not participate in the SENTINEL program. The SENTINEL and NON-SENTINEL Surveillance Systems cover 15 respiratory viruses, namely: influenza virus type A and B, RS virus type A and B, parainfluenza type 1, 2, 3, 4, human metapneumovirus (hMPV), adenovirus, rhinovirus, coronavirus 229E/NL63 and OC43/HKU1, and enterovirus. Since the onset of the SARS-CoV-2 pandemic, the SARS-CoV-2 virus has been included in the SENTINEL and NON-SENTINEL system reporting. Within the scope of epidemiological surveillance, VSES report weekly case numbers in 7 age groups: 0–4, 5–9, 10–14, 15–25, 26–44, 45–64, and over 65 years. Since the 2013/2014 season, all reporting has been conducted via an online platform (https://sentinel.pzh.gov.pl/artykuly/podglad.php?id_artykulu=29), where employees of NIC are the administrators and coordinators. They have access to the epidemiological and virological data entered by the employees of VSES. This continuous reporting supports global monitoring and ensures a rapid response if a new influenza subtype with pandemic potential emerges.

MATERIAL:

The analyses concerned data submitted to the SENTINEL system during the 2022/2023 and 2023/2024 epidemic seasons. Samples were collected from patients from all over the country. Swabs with transport medium or saline were used. During the 2022/2023 epidemic season, a total of 3324 samples were tested in the SENTINEL system and 5328 in NON-SENTINEL system. During the 2023/2024 epidemic season, 2613 samples were tested in the SENTINEL system and 10362 samples were tested in the NON-SENTINEL system.

Twice during the epidemic season (in January and April), VSES are required to send positive samples to the National Influenza Center-coordinator of the SENTINEL surveillance system. Further analyses are carried, including subtyping and assessment of the resistance of viruses to antiviral drugs (Oseltamivir and Zanamivir).

METHODS:

The research was conducted in VSES, hospital laboratories as well as in the laboratory of the NIC. NIC acting as a reference center, was responsible for controlling the analysis performed by VSES. The results were subsequently weekly entered into the SENTINEL system by VSES, along with annotations about the patient’s age, the testing method used, and the test outcome. Data results were downloaded weekly from the system and analyzed in NIC. Only NIC has access to the results entered to the system by VSES.

ANALYSIS PERFORMED IN VSES: The VSES do not use standardized testing protocols. Instead, they employ various IVD tests based on molecular biology methods, in accordance with the manufacturer’s recommendations (Table 1).

VIRAL RNA ISOLATION IN NIC:

The Maxwell 16 Viral Total Nucleic Acid Purification Kit (Promega Corporation, Madison, WI, USA) was used to isolate the genetic material in NIC. According to the recommendations of the manufacturer, 200 μL of the suspension was taken. An inventory of isolation reagents used in other laboratories was not prepared.

TYPING AND SUBTYPING INFLUENZA VIRUSES IN NIC:

In NIC primers and probes from the International Reagent Resource were used in the analyses. The SuperScript Platinum III (Seegene) was used for the analysis. Reactions were performed in a CFX OPUS 96 Real-Time PCR system under the following reaction conditions: RNA was subjected to reverse transcription (at 50°C for 30 minutes). The obtained DNA was subjected to the initial denaturation process (1 cycle at 95°C for 2 minutes), followed by 45 cycles of amplification denaturation at 95°C for 15 seconds, annealing at 55°C for 10 seconds and elongation at 72°C for 20 seconds. Overall volume of the reaction was 20 μL (5 μL of genetic material and 15 μL of mixed primers, probes, buffer, water, and enzyme). Positive controls were viruses that were selected for the vaccines:

STATISTICAL ANALYSIS:

The numbers and proportions of positive samples for different viruses (influenza virus type or subtype, influenza-like virus type) according to season (2022/2023 and 2023/2024) and surveillance system (SENTINEL, NON-SENTINEL) were analyzed using descriptive statistics methods.

Comparisons between data from both analyzed surveillance systems or the epidemiological seasons regarding the frequency of confirmation of particular viruses (in the case of influenza, also their types/subtypes) were conducted using the chi-square test. A significance level of 0.05 was assumed in all analyses.

Results

EPIDEMIC SEASON 2022/2023:

During the 2022/2023 epidemic season, a total of 3324 samples were tested in the SENTINEL system, of which 1047 (31.5%) were confirmed as positive for influenza infection, as shown in Figure 1. From all confirmed cases, 505 were determined to be influenza type A and were 351 influenza type B. Within detections of influenza type A, 92 cases were determined to be A/H1N1/pdm09 subtype and 99 were A/H3N2/subtype. In the NON-SENTINEL part of the surveillance system, 5328 samples were tested among the confirmed cases, and the following were registered: 1625 cases of unsubtyped type A viruses, 61 cases of A/H1N1/pdm09 viruses, 36 A/H3N2/, and 297 type B viruses. In both SENTINEL and NON-SENTINEL, influenza-positive cases display different percentages: influenza type A accounting for 69.5% of registered cases, influenza type B for 21.1%, subtype A/H1N1/pdm09: 5.0%, and A/H3N2/: 4.4%.

The NON-SENTINEL system more frequently confirmed influenza viruses (37.9% vs 31.5%), identified type A viruses (85.3% vs 66.5%), and recorded unsubtyped type A viruses (80.5% vs 48.5%). All differences were statistically significant (chi-square test, p<0.001).

INFLUENZA-LIKE VIRUSES DURING THE 2022/2023 EPIDEMIC SEASON:

During the 2022/2023 season, RSV was the predominant influenza-like virus, with 164 cases in the SENTINEL system and 66 in NON-SENTINEL, together accounting for 91.3% of detections (Figure 2). Other viruses, found only in SENTINEL, included adenovirus (13 cases), Mpv (4), rhinovirus (3), and PIV-4 (2). The overall detection rate was significantly higher in SENTINEL (5.6%) than in NON-SENTINEL (1.2%) (p<0.001).

EPIDEMIC SEASON 2023/2024:

During the 2023/2024 epidemic season, 2613 samples were tested in the SENTINEL system, with 874 (33.4%) confirmed cases: 536 unsubtyped type A viruses, 246 A/H1N1/pdm09, 35 A/H3N2/, and 57 type B viruses (Figure 3). In the NON-SENTINEL system, 10 362 samples were tested, with 5872 confirmed cases: 4749 unsubtyped type A viruses, 641 A/H1N1/pdm09, 18 A/H3N2/, and 464 type B viruses. The dominant virus types were unsubtyped type A (78.3%), A/H1N1/pdm09 (13.1%), type B (7.7%), and A/H3N2/ (0.8%).

The NON-SENTINEL system more frequently confirmed influenza viruses (56.7% vs 33.4%) and recorded unsubtyped type A viruses (80.9% vs 61.3%). The differences were statistically significant (chi-square test, p<0.001).

INFLUENZA-LIKE VIRUSES DURING THE 2023/2024 EPIDEMIC SEASON:

During the 2023/2024 epidemic season, a total of 841 cases of influenza-like viruses were confirmed across the SENTINEL and NON-SENTINEL systems (Figure 4). The dominant pathogen was SARS-CoV-2, accounting for 87.6% of all detections. In the SENTINEL system, 723 cases were confirmed, with SARS-CoV-2 (86.6%) and RSV (11.5%) being the most common. Other viruses included PIV-4 (5 cases), Rh-4 and Adv (4 each), and Mpv (1 case). In the NON-SENTINEL system, 111 cases were confirmed, with SARS-CoV-2 comprising 94.1%, followed by RSV (2.5%), Rh (1.7%), and both PIV-4 and Mpv (0.8%).

A statistically significant difference was observed in detection rates between systems (p<0.001), with 27.7% positivity in SENTINEL vs 1.1% in NON-SENTINEL. Additionally, the proportion of SARS-CoV-2 among confirmed cases was significantly higher in the NON-SENTINEL system (94.1% vs 88.6%, p=0.022).

COMPARISON OF THE 2022/2023 AND 2023/2024 SEASONS:

In the 2023/2024 season, the number of samples tested decreased by 21.4% in the SENTINEL system, while it nearly doubled (+94.5%) in the NON-SENTINEL system. Despite this, the detection rate of influenza viruses in SENTINEL remained stable (33.4% vs 31.5% in 2022/2023), whereas in NON-SENTINEL it increased significantly (56.7% vs 37.9%; p<0.001).

For influenza-like viruses, the trend was reversed: SENTINEL showed a marked increase in detection (from 5.6% to 27.7%; p<0.001), mainly due to SARS-CoV-2, while NON-SENTINEL remained virtually unchanged (1.2% vs 1.1%).

The structure of detected influenza viruses also shifted. In both systems, the share of type A viruses increased significantly (SENTINEL: 66.5% → 93.5%; NON-SENTINEL: 85.3% → 92.1%; p<0.001), reducing the proportion of type B viruses. The share of unsubtyped type A viruses rose in SENTINEL (48.5% → 61.3%; p<0.001) but remained stable in NON-SENTINEL (80.5% → 80.9%). However, their relative share within type A viruses decreased in both systems (SENTINEL: 72.6% → 65.6%, p=0.004; NON-SENTINEL: 94.4% → 87.8%, p<0.001).

As in previous years, the peak of infections occurred between January and March, with the highest number of confirmed influenza cases recorded in week 2 of 2022/2023 and week 8 of 2023/2024 (Figure 5).

Discussion

LIMITATION OF THE STUDY:

The work is based solely on the analyses of samples that were reported to the sentinel influenza surveillance system. Not all tested samples are reported to the system; therefore, the number of patients studied in Poland in given seasons could be much higher.

Conclusions

The influenza surveillance system in Poland enables continuous, weekly monitoring of the epidemiological situation throughout each season. The SENTINEL and NON-SENTINEL systems, despite methodological differences, provided complementary insights into virus circulation patterns. This allows for a rapid response in the event of the emergence of new influenza virus variants or other respiratory viruses. The results obtained as part of influenza surveillance in Poland are consistent with those observed in other European countries. Influenza is a huge threat, especially for patients from high-risk groups, and causes many deaths every season. The best way to prevent influenza is seasonal influenza vaccination, which is why the level of vaccination should be increased.