The Association of Seasonal Variations and COVID-19 Clinical Features: A Comparative Study on the Fourth and Fifth Waves

Purpose The COVID-19 pandemic has overwhelmed many healthcare systems. Seasonality is a feature of several infectious diseases. Studies regarding the association of seasonal variations and COVID-19 have shown controversial results. Therefore, we aimed to compare COVID-19 characteristics and survival outcomes between the fourth and fifth waves in Iran, which corresponded to spring and summer, respectively. Methods This is a retrospective study on the fourth and fifth COVID-19 waves in Iran. One hundred patients from the fourth and 90 patients from the fifth wave were included. Data from the baseline and demographic characteristics, clinical, radiological, and laboratory findings, and hospital outcomes were compared between the fourth and fifth COVID-19 waves in hospitalized patients in Imam Khomeini Hospital Complex, Tehran, Iran. Results The fifth wave patients were more likely to present with gastrointestinal symptoms than the patients from the fourth wave. Moreover, patients in the fifth wave had lower arterial oxygen saturation on admission (88% vs. 90%; P = 0.026), lower levels of WBCs (neutrophils and lymphocytes) (6300.00 vs. 8000.00; P = 0.004), and higher percentages of pulmonary involvement in the chest CT scans (50% vs. 40%; P < 0.001). Furthermore, these patients had longer hospital stays than their fourth-wave counterparts (7.00 vs. 5.00; P < 0.001). Conclusions Our study indicated that patients in the summer COVID-19 wave were more likely to present with gastrointestinal symptoms. They also experienced a more severe disease in terms of peripheral capillary oxygen saturation, percentages of pulmonary involvement in CT scans, and length of hospital stay.


Introduction
COVID-19 has emerged as a pandemic since its frst reports in late December 2019 [1]. Te disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has overwhelmed many healthcare systems worldwide [2]. According to the World Health Organization (WHO), a total of 380 million COVID-19 cases have been reported worldwide, including 5,680,741 deaths up to February 3 rd , 2022. In the same period, there have been over 6.4 million confrmed cases of COVID-19 in Iran, including 132,000 reported deaths [3].
To the best of our knowledge, this is the frst study on the Iranian population to explore the seasonal patterns of COVID-19 symptoms and severity. We aimed to compare COVID-19 characteristics between the fourth and ffth pandemic waves (corresponding to spring and summer, respectively). Tus, we investigated the association of COVID-19 clinical, laboratory, and radiological characteristics and outcomes with the seasonal changes in an observational study on the Iranian population.

Materials and Methods
Tis is a retrospective observational study. We used the data from two timeframes of March and June 2021, corresponding to the fourth and ffth COVID-19 waves in Iran, respectively. We selected one hundred ninety adult patients (100 and 90 from the fourth and ffth waves, respectively), aged over 18 years with available medical records, who were admitted to Imam Khomeini Hospital Complex during the study timeframe by a simple random sampling method from all COVID-19 patients. Patients undergoing outpatient management or patients with a hospital stay shorter than 24 hours or with incomplete medical records were excluded from the study. Te protocol of this study was reviewed and approved by the Ethics Committee of the Tehran University of Medical Sciences (Registration no. IR.TUMS.IKH-C.REC.1400.395). A code was assigned to each participant, and all data analyses were performed anonymously.
Data regarding baseline and demographic characteristics, clinical characteristics, radiological and laboratory fndings, and hospital outcomes were compared between the fourth and ffth COVID-19 waves in hospitalized patients in Imam Khomeini Hospital Complex, Tehran, Iran.

Confrmation of Diagnosis.
Te COVID-19 diagnosis was confrmed based on defnite evidence of COVID-19 pulmonary involvement in a spiral chest computed tomography (CT) scan or positive reverse transcription polymerase chain reaction (RT-PCR) conducted on pharyngeal swabs.

Radiological and Laboratory Findings.
We cited radiologists' reports of pulmonary involvement (the presence and pattern of involvement, including ground-glass opacity (GGO) or consolidation, and the quantity of involvement reported as a percentage) from spiral CT scans of the chest. Laboratory fndings such as complete blood count (CBC) with diferentiation were retrieved from patients' medical records.

Hospital
Outcomes. Hospitalization-related data, including the mode of respiratory support, length of hospital stay, and in-hospital deaths, were collected from the medical records.

Statistical Analysis.
We carried out the Kolmogorov-Smirnov test to explore whether the data distribution was normal. We reported the mean ± standard deviation (SD) for normally distributed data and the median (interquartile range) for non-normally distributed data. We performed the independent sample T-test to compare quantitative variables, the Chi-square test to compare qualitative variables, and the Mann-Whitney U test to compare the non-normally distributed data between the two groups. All statistical analyses were conducted by the Statistical Package for Social Sciences software (SPSS Inc. Version 26). P values less than 0.05 were considered statistically signifcant.

Results
A total of 190 patients (100 from the fourth and 90 from the ffth COVID-19 waves) were included in the study. Te overall characteristics of study participants are shown in Table 1. Table 2 summarizes the baseline characteristics of the study participants. Hospitalized COVID-19 patients in the ffth wave had a signifcantly higher prevalence of smoking than those in the fourth wave. In contrast, the two waves did not difer signifcantly regarding the mean, age, and gender distribution of patients. In addition, patients' other comorbidities and their prevalence were not signifcantly diferent between the two waves. Table 3 describes the clinical, laboratory, and radiological characteristics of patients in the fourth and ffth waves of COVID-19. Patients were more likely to experience cough and gastrointestinal symptoms, including nausea, vomiting, diarrhea, and constipation, during the ffth wave. Patients in the ffth wave presented with a more extensive pulmonary involvement in the spiral CT scan than those in the fourth wave (U = 5867, P < 0.001). Furthermore, the pattern of radiological fndings was diferent between the two waves (U = 3965, P � 0.002). In addition, the Mann-Whitney U test showed a signifcantly lower SpO2 at the time of   Table 4 illustrates the hospital outcomes of COVID-19 patients in the fourth and ffth waves. Te Mann-Whitney test showed that the mode of respiratory support signifcantly difered between the fourth and ffth waves (U = 6148.5, P < 0.001). Although patients in the ffth wave required a longer hospital stay (7.00 (6) vs. 5.00 (3), P < 0.001), they were not signifcantly diferent from the patients in the fourth wave regarding in-hospital deaths.

Discussion
Tis study, to the best of our knowledge, is the frst study on the Iranian population aiming at the comparison of COVID-19 characteristics between the fourth and ffth waves (corresponding to spring and summer 2021, respectively). We investigated the potential association of COVID-19 clinical, laboratory, and radiological features and outcomes with seasonal changes.
Our results indicate that patients in the ffth wave (corresponding to the Delta variant of COVID-19) had a lower arterial oxygen saturation on admission, lower levels of WBCs (neutrophils and lymphocytes), and higher percentages of pulmonary involvement in the chest CT scans. Moreover, these patients had longer hospital stays than their fourth-wave counterparts. In other words, Iran's summer wave (ffth wave) had a more severe clinical phenotype with a more severe pulmonary involvement. Since comorbidities were comparable between the two waves, the more severe clinical feature in the ffth wave, in part, can be explained due to the dominant Delta variant and the potential disease seasonality. A study has shown that the Delta variant is highly transmissible, accounting for potentially more severe diseases [20]. In contrast, other studies have not confrmed this increase in disease severity during Delta-predominant waves [21][22][23]. Patients in the ffth wave were more likely to be managed by noninvasive ventilation and less likely to be intubated despite the more severe disease and pulmonary involvement. We hypothesize that higher experience in COVID-19 management could have led to a higher threshold for endotracheal intubation in the ffth COVID-19 wave in Iran.
Several studies have compared COVID-19 waves worldwide from the frst wave till now. Tese comparative studies have yielded inconsistent results, even within one country [24]. In a study conducted in the United States, new COVID-19 cases were positively linked with temperature and humidity [25]. A modeling study in the United States suggested that the increase in temperature and humidity decreases COVID-19 spread [26]. In other studies, researchers have suggested that the rise in temperature decreases the risk of COVID-19, but higher levels of humidity increase the disease risk [27,28]. Studies from Italy have compared the clinical features and outcomes of the frst and second COVID-19 waves. Despite some conficting results, their results showed that the second-wave patients experienced a less severe disease with better outcomes [29,30]. In a cohort study conducted in South Africa, the second wave was associated with higher hospital admissions and increased mortality than the frst wave [31]. In studies performed in India, the patients in the second wave had more comorbidities [32] and experienced a greater severity of disease [33], and their mortality was signifcantly higher than their frst-wave counterparts [32,34]. A comparative study on the frst and second COVID-19 waves in southern Germany showed improved survival outcomes in the second wave. Tis survival improvement was mainly observed among patients requiring intensive care and mechanical ventilation. It corresponded to the more frequent use of nasal high-fow (NHF) oxygen and noninvasive ventilation (NIV) instead of intubations [35]. Another study from Germany indicated that although in-hospital mortality did not difer between the frst and second waves, patients in the second wave were more likely to be treated as outpatients and had a signifcantly shorter duration of hospitalization [36]. Tese inconsistent fndings highlight the importance of country-specifc and region-specifc data when comparing the COVID-19 waves.
Te fourth and ffth COVID-19 waves in Iran corresponded to spring and summer, respectively. Our results revealed that in the ffth wave, which occurred in the summer, gastrointestinal symptoms were the predominant feature of COVID-19. Accordingly, it is implied that the pattern of COVID-19 presentation may difer across seasons and probably regions and countries [37]. Terefore, we believe that implementing identical diagnostic criteria across diferent seasons and geographical locations appears inaccurate. Such potentially insensitive criteria can lead to underdiagnoses and unsuccessful break of the chain of transmission during the COVID-19 pandemic. Our fndings suggest that the clinical criteria for COVID-19 diagnosis should be modifed according to seasons and locations. Tis spatiotemporal approach to altering the diagnostic criteria would contribute to timely case fndings, followed by appropriate restrictive measures.
Diagnostic testing for COVID-19 is crucial in the prevention and control strategy [38]. To achieve this goal, we suggest that patients with nonspecifc COVID-19 symptoms, considering seasonal variabilities, undergo screening tests using rapid antigen testing in the frst place. WHO recommends rapid antigen testing as a screening tool for COVID-19 in individuals who have been in contact with COVID-19 patients, primarily those with an increased risk of developing severe disease or those with high levels of exposure [38,39]. According to the WHO's interim guidance on 'Antigen-detection in the diagnosis of SARS-CoV-2 infection,' updated in October 2021, these tests are more reliable in regions with ongoing community transmission, defned as a test positivity rate of at least 5% [38]. Rapid antigen tests provide results in several minutes, which is signifcantly shorter than RT-PCR [38,40]. Tis shorter interval is critical because a vital driver of viral spread is presymptomatic or paucisymptomatic transmission [41,42]. Terefore, rapid antigen tests provide the opportunity for timely diagnosis and interruption of disease transmission [38]. If rapid antigen tests are positive, patients will undergo the more accurate but time-consuming diagnostic testing, i.e., RT-PCR. In case of positive RT-PCR, these patients will undergo containment measures, including quarantine and close contact testing, to prevent viral spread.
By February 2022, Iran has entered the sixth COVID-19 peak, when the Omicron variant is dominant. Keeping in mind that this peak will possibly become established in the winter, we suggest that further investigations be made regarding the pattern of COVID-19 presentations. Te fndings of this study and the data from the potential winter peak can be used to provide a more comprehensive comparison of COVID-19 manifestations across diferent seasons.
Our study had several limitations. First, it was an observational, retrospective study with a modest sample size conducted in one hospital. Although our hospital is a tertiary referral center where patients are admitted from all regions of Iran, the single-center nature of our study could potentially limit the generalizability of our fndings. Second, whether the Delta variant of SARS-CoV-2 was responsible for the disease severity in the ffth wave was not documented by diagnostic tests of variances in our hospital because these tests were not conducted for all patients at our hospital. Terefore, the predominance of this variant in our hospital was implied by the predominance of the Delta variant in Iran's ffth wave [43].

Conclusions
In conclusion, we observed that patients in the ffth COVID-19 wave, which corresponded to summer in Iran, were more likely to present with gastrointestinal symptoms. In addition, they had lower oxygen saturation on admission and higher percentages of pulmonary involvement in CT scans. Despite the longer duration of hospitalization in the ffth wave, patients did not difer signifcantly regarding in-hospital mortality compared to patients in the fourth wave. Based on our fndings, we suggest that the clinical diagnostic criteria for COVID-19, even in the same region and hospital, should  Severe acute respiratory syndrome coronavirus 2 CT: Computed tomography RT-PCR: Reverse transcription polymerase chain reaction DM: Diabetes mellitus SpO 2 : Peripheral capillary oxygen saturation GGO: Ground-glass opacity WBC: White blood cell CBC: Complete blood count HTN: Hypertension IHD: Ischemic heart disease CKD: Chronic kidney disease IQR: Interquartile range SD: Standard deviation NIV: Noninvasive ventilation.

Data Availability
Te datasets generated or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.

Ethical Approval
Te protocol of this study, in accordance with the principles of the Declaration of Helsinki, was reviewed and approved by the Ethics Committee of the Tehran University of Medical Sciences (Registration no. IR.TUMS.IKHC.REC.1400.395). A code was dedicated to each participant, and all data analyses were retrospectively and anonymously performed.

Conflicts of Interest
Te authors declare that they have no conficts of interest.