Understanding the High Incidence of Pertussis in Norway: The Role of Test Activity

Pertussis (whooping cough) is a highly contagious respiratory infection caused by the bacterium Bordetella pertussis. Infants have the highest risk of severe disease and, in rare cases, may die from the disease. Pertussis vaccines were available from the 1940s and have become cornerstones in infant combination vaccines worldwide. Norway introduced the pertussis vaccine as a component of the combination vaccine against diphtheria, tetanus and pertussis (DTP) in 1952, switching from the whole cell pertussis vaccine to an acellular vaccine in 1998. Pertussis vaccination is now offered according to a 2 + 1 schedule at 3, 5 and 12 months of age, with school-age boosters at 7 and 15 years. Maternal vaccination against pertussis was introduced in May 2024, and is offered to all pregnant women in gestational week 24 [1]. Since 1998, Norway has reported the highest number of pertussis cases per 100 000 inhabitants in Europe nearly every year [2]. Nevertheless, the disease burden in infants has remained low, and vaccination coverage is sustained at a high level (> 95%). The high incidence of cases has raised questions about potential differences in testing practices between countries. Testing for B. pertussis by PCR is widely available and often included in clinical diagnostic panels for respiratory infections, although the pathogens included in these panels may vary between laboratories. All cases of pertussis have been notifiable to the Norwegian Surveillance System for Communicable Diseases (MSIS) since 1993. Since 2012, the majority of laboratory-confirmed pertussis cases have been diagnosed by PCR, reaching 97% in 2024. Since September 2020, all PCR test results for B. pertussis have been reported to the MSIS laboratory database, enabling the surveillance of test activity and providing the opportunity to assess whether the high incidence of pertussis in Norway is associated with high test activity. Between 2000 and 2019, the incidence of pertussis in Norway varied between 61 and 146 per 100 000 inhabitants [3]. After practically no circulation of B. pertussis in 2021–2022 during the COVID-19 pandemic when strict public health and social measures were in place, the incidence of pertussis began to increase in the second half of 2023. In May 2024, the Norwegian Institute of Public Health (NIPH) declared a national outbreak of pertussis, as did other European countries [4]. In 2024, 10 209 cases of pertussis were reported and represented the highest annual incidence in Norway in the 21st century (184 per 100 000). Children aged 5–14 years were most affected by the outbreak (574 per 100 000), followed by the age groups 0–4 years (232 per 100 000) and 15–29 years (196 per 100 000). Older adults were not left unaffected: 121, 80 and 68 cases per 100 000 were reported in the age groups 30–64, 65–79 and 80 years or older, respectively. Information on which infection is suspected is not reported to the NIPH together with test results. When a large outbreak of Mycoplasma pneumoniae infection coincided with the pertussis outbreak, the NIPH recommended testing individuals with a heavy cough for both B. pertussis and M. pneumoniae [5]. Consequently, more than 455 000 samples (> 8100 per 100 000 inhabitants) were analysed for B. pertussis with PCR in 2024, presumably representing the highest test activity ever seen for B. pertussis in Norway. Among children and adults < 65 years, most tests were performed in outpatient settings, while the proportion of tests from hospitalised patients increased in the ≥ 65 year age group and reached almost 50% in the elderly ≥ 80 years. As expected, children < 1 year of age and 1–4 years of age were tested most frequently (approximately 18 000 samples and > 11 000 samples per 100 000, respectively), closely followed by those aged ≥ 80 years (> 11 000 samples per 100 000). The test frequency for B. pertussis increased in other age groups, too, with the number of samples analysed per 100 000 ranging from approximately 7300 among 30–64-year-olds to 8600 among 5–14-year-olds. During the 2024 outbreak, the number of pertussis cases hospitalised at the time of notification was the highest since 2000, with 228 patients (2% of all notified cases) recorded in MSIS. During the last large epidemic in Norway which occurred in 2004, 6697 cases (146 per 100 000) were reported, with 113 hospitalisations (2% of all notified cases) (Table 1). Of these, 71% were under the age of 1 year (80 cases; 58% of all notified cases in this age group), including one infant death. In 2024, 31 infants were admitted to hospital, representing 14% of all admissions, and 24% of all cases notified in this age group. There were no reported infant deaths. The relatively low proportion of infant hospitalisations in relation to other age groups may be explained by increased testing in other age groups, especially among hospitalised individuals, although the association to maternal vaccination status also likely contributes but has not yet been investigated. Laboratories detecting a positive PCR for any Bordetella species are requested to send the sample to the National Reference Laboratory at the NIPH for culture and further investigations. In 2024, a total of 2518 samples were sent, representing a more than a 10-fold increase compared to 2023. Of the 504 samples that could be cultured, 97% (n = 488) were identified as B. pertussis and 3% were other Bordetellas (n = 16). In comparison, in 2023, 39% of the 104 samples that could be cultured were identified as B. holmesii or other Bordetella species (n = 41) while 61% were B. pertussis (n = 63). These results indicate that the high incidence of pertussis cases in 2024 reflect a true increase in B. pertussis circulation, at the expense of other Bordetella species, which cause less severe disease. Testing in all age groups provides a better understanding of the pertussis disease burden in the population. However, it is important to acknowledge that information on the indication for testing is not available, and testing practices may be influenced by concurrent outbreaks of other respiratory diseases with similar clinical pictures. This complicates the interpretation of the findings and incidence should thus be evaluated in the context of test activity. Being able to estimate a positivity rate based on positive and negative test results in the population improves the understanding of the disease epidemiology. Our surveillance demonstrates a moderate disease burden in adults and the elderly. This information is useful when forming vaccine recommendations for these age groups, specifically the necessity and timing of adult booster doses. Norway is currently establishing an adult vaccination programme, though booster doses against pertussis are not yet planned to be included. In conclusion, easy access to testing for B. pertussis and high test activity may have contributed to the high incidence of pertussis recorded in 2024. While Norway did experience a true outbreak of pertussis in 2024, comparing the size of this outbreak to previous ones is difficult due to the lack of historical data on test activity for previous epidemic years. As the current pertussis vaccines do not provide lasting protection against disease, high test activity may contribute to controlling epidemics since treatment is initiated upon positive test results and further spread is halted. Future research should assess the cost-effectiveness of high test activity to control pertussis outbreaks. Ragnhild Bardal Roness and Håkon Bøås for invaluable input on the manuscript. According to the Act relating to the control of communicable diseases § 7–9, the Norwegian Institute of Public Health (NIPH) has the mandate to monitor the national epidemiological situation for infectious diseases. The NIPH can handle health and other information without informed consent or ethical approval for surveillance purposes. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.