2026 Volume 72 Issue 2 Pages 183-187
The objective of this study was to investigate the trends in the number of positive cases of influenza and dengue fever before and during the COVID-19 pandemic in a private hospital in Cambodia. This study was a preliminary observational prospective descriptive study. A total of 2,991 patients were included. The positivity rate for dengue fever was higher before COVID-19 (14.7%, 179/1217) than during the COVID-19 period (4.1%, 10/242). For influenza, the positivity rates before COVID-19 were 13.3% (185/1394) for influenza A and 10.7% (149/1394) for influenza B. During the COVID-19 period, these rates changed to 18.2% (174/958) for influenza A and 0.4% (4/958) for influenza B. A whole country assessment of dengue and influenza disease patients should be done during the COVID-19 pandemic term.
Dengue fever is a globally neglected infectious disease, with more than 6.5 million people infected and 7,300 deaths reported in 2023. The number of dengue fever cases reported to the WHO has increased sharply over the past few decades, rising from 505,430 cases in 2000 to 5.2 million in 20191). Seasonal influenza affects approximately one billion people annually, with 290,000 to 650,000 deaths2). According to the WHO, 99% of influenza-related deaths in children under five occur in developing countries2). Identifying the factors that influence the rise and fall of diseases like dengue fever and influenza is a critical public health challenge.
To decrease the number of dengue fever cases, various measures have been implemented. For instance, strategies to reduce contact with mosquitoes include wearing clothing that covers the body as much as possible, using mosquito nets, window screens, and repellents. To prevent mosquito breeding, measures such as eliminating places where water could pool, regularly emptying water storage containers, and applying insecticides to these containers have been employed3). For influenza, vaccination is the most effective measure. Additionally, regular handwashing, covering the mouth and nose when coughing or sneezing, avoiding going out, minimizing contact with unwell individuals, and refraining from touching the eyes, nose, and mouth are recommended preventive practices4). Furthermore, infection control measures during the COVID-19 pandemic, such as handwashing, social distancing, and wearing masks, overlap significantly with those for influenza prevention. Previous studies have reported that these measures helped reduce the number of influenza cases5). However, limited research has been conducted comparing the trends in dengue fever and influenza cases before and after the COVID-19 pandemic.
Dengue fever is endemic throughout Cambodia, with large-scale outbreaks occurring every few years. In the past 20 years, the dengue fever pandemic occurred in 2007, 2012, and 20196). Infection peak was during the rainy season between June and August;and this occurred both dengue pandemic year and non-pandemic year in Cambodia6). Influenza infections also occur year-round in Cambodia7). On the other hand, the Cambodian government successfully controlled the spread of COVID-19 with a multifaceted approach that included training infection control teams, dispatching them to various regions, and implementing lockdowns8). Thus, Cambodia is an ideal region for analyzing trends in dengue fever and influenza case numbers before and after the COVID-19 pandemic.
The objective of this study was to investigate the trends in the number of positive cases of influenza and dengue fever before and during the COVID-19 pandemic.
Sunrise Japan Hospital Phnom Penh was a private general hospital established in 2016 in Phnom Penh. The hospital provides a wide range of medical services, including emergency care, surgeries, and outpatient consultations. It has 50 beds, 15 medical departments, and various facilities, such as an intensive care unit (ICU), inpatient beds, and vascular treatment capabilities. The hospital aims to deliver high-quality medical care adhering to Japanese standards in Cambodia. Most of its patients are local people.
Study designThis study was a preliminary observational prospective descriptive study conducted at Sunrise Japan Hospital Phnom Penh, in Cambodia. It observes the changes in the number of positive test results for influenza and dengue fever before and during the COVID-19 pandemic. This study was approved by the National Ethics Committee of Cambodia (approval number:20200921-2).
Participants and assayThe observation period ranged from May 2016 to October 2021. Testing was performed on patients who visited the outpatient department or were hospitalized at SJH and presented symptoms related to dengue fever or influenza, requiring testing for confirmation or exclusion of infection. The testing kits used were Bioline Dengue Duo for dengue fever and Bioline Influenza Ag for influenza (Abbott)9,10). The tests were primarily performed by clinical nurses. Basic information, such as age and sex, was obtained from the patients’ medical records.
Statistical analysisA descriptive analysis was conducted to observe changes in the number of positive test results for influenza and dengue fever among outpatient and inpatient populations before the COVID-19 pandemic (May 2016-December 2019) and during the pandemic (January 2020-October 2021). The Chi-square test was performed for categorical variables, and the t-test was performed for continuous variables. Events related to COVID-19, such as the first diagnosed case in Cambodia and trends in COVID-19 case numbers, were descriptively analyzed in relation to the trends in dengue fever and influenza cases.
A total of 2,991 patients were included in the final analysis. The number of positive COVID-19 cases was shown in Supplemental Figure 1. The number of COVID-19 cases increased dramatically after 2021 (Supplemental Figure 1). The missing data number for each valuable was shown in Supplemental Table 1. Before the COVID-19 pandemic, the proportion of tests conducted was higher during the rainy season (May to October). However, this trend disappeared during the COVID-19 period (Table 1).
The positivity rate for dengue fever was higher before COVID-19 (14.7%, 179/1217) than during the COVID-19 period (4.1%, 10/242). For influenza, the positivity rates before COVID-19 were 13.3% (185/1394) for influenza A and 10.7% (149/1394) for influenza B. During the COVID-19 period, these rates changed to 18.2% (174/958) for influenza A and 0.4% (4/958) for influenza B. Cases of influenza A continued to be reported even after the first COVID-19 case. There was no significant difference in the proportion of sexes between the periods before and during COVID-19 (p=0.072). However, the mean age of patients who received dengue or influenza testing was lower during the COVID-19 period (p < 0.001, Table 1).
The number of dengue fever tests was higher before the first reported COVID-19 case. After the emergence of COVID-19, the absolute number of dengue tests decreased. Notably, following the implementation of mass gathering restrictions in April 2020, the number of dengue fever tests dropped to zero in the subsequent month. From February to July 2020, no positive dengue cases were detected, although a few cases were reported beginning in August 2020. However, the number of positive cases remained significantly lower compared to the pre-pandemic period (Figure 1A). Besides, focusing on non-pandemic years of dengue disease (except around 2019)6), the positive dengue disease cases during the COVID-19 pandemic (23 months) were 7 cases, yet the positive dengue disease cases during the non-COVID-19 period (22 months, November 2016-August 2018) were 33 cases;the positive case seems to be high in non-COVID-19 pandemic terms.
The number of influenza tests exhibited three peaks:around July 2019, February 2020, and October 2020. The number of influenza A positive cases corresponded with these peaks in testing. In contrast, for influenza B, there was a single peak before the pandemic;however, almost no positive cases were detected during the pandemic. Unlike dengue fever, influenza A positive cases continued to be observed after the first reported COVID-19 case (Figure 1B).
+ : Chi-squared test was performed for the categorical variable, t-test was performed for the continuous variable
Relationships between the number of patients with positive or negative results of dengue disease or influenza testing and events associated with COVID-19
(A) dengue disease (B) influenza
Studies comparing trends in the number of dengue fever and influenza cases before and after the COVID-19 pandemic were limited. This study analyzed trends in the proportion of positive test results for influenza and dengue fever before and after the first reported COVID-19 case in a private hospital in Cambodia’s capital.
The positivity rates for dengue fever and influenza B testing decreased after the onset of the COVID-19 pandemic, while influenza A cases continued to be detected. As shown in Figures 1 and 2, after the pandemic began, cases of dengue fever and influenza B were almost non-existent. Previous studies reported similarities in the transmission routes of COVID-19 and influenza and found that strengthened infection control measures due to COVID-19 led to a reduction in influenza cases. This finding was consistent with the observed decline in influenza B cases in this study5). To fully understand the impact of COVID-19 control measures on the incidence of specific diseases, a comprehensive assessment of how these measures influenced the reduction of other infectious diseases was required.
After the first case of COVID-19 was reported, the total number of tests for influenza and dengue fever decreased, and the traditional seasonal trend of increased testing during the rainy season disappeared. Before the pandemic, 69.3% of tests were conducted between May and October;however, this dropped to 37.4% after the first case of COVID-19. Previous studies have also reported the change in the incidence of various infectious diseases during the COVID-19 pandemic5). Yet, the total number of dengue disease patients was limited in our study;the seasonal peaks may not appear in non-pandemic years of dengue disease. A whole country assessment of dengue disease patients should be done during the COVID-19 pandemic term.
This study has several limitations. First, the present study was a single-center study; generalizing the findings to other regions had several challenges. Second, the small sample size might have caused chance findings in the study results through random error. The random error might have influenced the observed positive proportion of dengue and influenza infection, especially during the COVID-19 pandemic term with a small sample size. Third, the information on participants’ backgrounds was limited. Fourth, the mean age was significantly lower during the COVID-19 pandemic;the middle-aged to aging population may have refrained from visiting hospitals. Fifth, the overall number of patients was small;the seasonal peaks in non-dengue pandemic years may not appear. Sixth, it was unclear whether the decrease in detected dengue disease cases stems from reduced testing or a genuine reduction in disease incidence resulting from government restrictions. Despite these limitations, this study was the first longitudinal analysis study in Cambodia to examine the testing trends for dengue fever and influenza before and after the COVID-19 pandemic.
The positivity proportion of dengue fever and influenza B testing dramatically decreased after the first case of COVID-19. Adequate implementation of infection testing was required, even during the emergency phase of the pandemic.
This research did not receive any specific grant from any funding agency in the public, commercial, or non-profit sector.
All the authors report no proprietary or commercial interest in any product mentioned or concept discussed in this article.
We would like to thank all staff at the Sunrise Japan Hospital in Phnom Penh for providing critical data for this research.
This study was approved by the National Ethics Committee for Health Research in Cambodia.
All the authors made a substantial contribution to this research. BK, TT, YK, and MO contributed to writing the paper;BK, YK, and MO contributed to the study design, data collection, and coordination.
