Comparison of COVID-19 Prevalence and Clinical Presentation During Pre- and Post-Vaccination Eras in Iran

1. INTRODUCTION

The coronavirus disease 2019 (COVID-19) has had a profound impact on global health since its emergence in late 2019. As a highly contagious viral infection caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), COVID-19 has led to significant morbidity and mortality worldwide [1]. During the course of the disease, individuals may experience a range of symptoms, from mild to severe, with asymptomatic cases also being reported [2]. Common symptoms include fever, cough, fatigue, sore throat, and loss of taste or smell [3]. In severe cases, the disease can lead to complications such as pneumonia, Acute Respiratory Distress Syndrome (ARDS), multiple organ failure, and death [4]. Older adults and individuals with pre-existing chronic diseases are at a higher risk of severe illness and death [5].

Since the onset of the pandemic, several variants of SARS-CoV-2 have emerged with varying degrees of transmissibility, virulence, and consequences. Some variants, such as Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), and Omicron (B.1.1.529), were classified as variants of concern [6, 7]. The clinical manifestations of COVID-19 have evolved. Initially, the prevalence of taste or smell loss was reported to be high; however, the frequency of this symptom decreased afterwards, and upper respiratory symptoms, such as sore throat and cough, increased in prevalence [8]. It has been reported that different variants of SARS-CoV-2 may be associated with specific symptoms. For instance, the wild-type variant was associated with symptoms such as fever, respiratory and gastrointestinal manifestations, and loss of taste and smell. In contrast, the Omicron variant was associated with symptoms including a sore throat and headaches [9]. Recent studies confirm that Omicron BA.1/BA.2 subvariants disproportionately cause upper respiratory symptoms compared to earlier variants [10].

Furthermore, the predominant clinical symptoms were influenced by the introduction of different types of vaccines against SARS-CoV-2. As more individuals are vaccinated, the overall severity of the disease is expected to decrease, resulting in a milder course for most infected individuals [11]. Post-vaccination behavioral changes, such as reduced testing of mild cases and altered healthcare-seeking behaviors, may also introduce bias to symptom profiles. Therefore, further research is necessary to monitor changes in the clinical features of SARS-CoV-2 infection and to predict future shifts in the virus's behavior.

Iran was among the first countries to experience the quick spread of the SARS-CoV-2 infection. Since the first confirmed case of COVID-19 in February 2020, Iran has experienced seven outbreak waves of this infection up to late summer 2022 [12]. To the best of our knowledge, a comparison of the clinical presentation of different SARS-CoV-2 variants has not been previously reported among the non-hospitalized population in Iran. This cross-sectional study aimed to evaluate the prevalence and risk factors of COVID-19, as well as the associated clinical manifestations, among the Iranian population during different epidemic periods. We emphasized the changes in COVID-19 epidemiology following the country’s widespread national immunization against the infection after the fifth wave, which was primarily caused by the delta variant in Iran.

2. MATERIAL AND METHODS

Among the available clinical records of 85,262 individuals referred to the Central Laboratory of ACECR, Razavi Khorasan Branch, Mashhad, Iran, we had access to the clinical records of 85,262 individuals who were referred to the Central Laboratory of the ACECR, Razavi Khorasan Branch, Mashhad, Iran. Using pilot data, we estimated a 45% prevalence rate for each wave of SARS-CoV-2 infection in our population. Considering a 95% confidence coefficient (α = 0.05) and a 5% prevalence rate as the confidence interval (d = 0.0225), we needed to enroll 1878 people in the survey for each of the third to sixth waves of the pandemic. Thus, we calculated a total sample size of 7512 cases and attempted to select 6-10% of the total population for each wave. Using a systematic random sampling method, we selected a sample of 7051 individuals who were referred between September 2020 and March 2022 (Fig. 1, Table 1). We considered the last COVID-19 wave as the post-vaccination period, as it occurred at a time when widespread national immunization had been implemented in Iran. By January 2022 (onset of the sixth wave), approximately 70% of Iran’s eligible population had received a minimum of two doses of the Sinopharm, AstraZeneca, or COVIran Barekat vaccines [13]. Individual vaccination status was unavailable in the registry. The presence of chronic diseases, including cardiovascular, hypertension, kidney disorders, respiratory diseases, diabetes, obesity, transplantation, malignancy, stroke, and sickle cell anemia, was assessed by a self-reported survey. The assessment of symptoms was conducted through the implementation of structured questionnaires, which were administered during the course of clinical visits.

2.1. Data Collection Method

The records encompassed demographic data, such as age and gender, as well as a self-reported history of underlying chronic disease and COVID-19-associated clinical symptoms, including fever, chills, sore throat, cough, dyspnea, and loss of taste or smell. Two nasopharyngeal swab samples were collected from each participant by trained staff and transferred to a Viral Transport Medium (VTM) solution tube. SARS-CoV-2 RNA was extracted from the samples (MedNet EC-REP GmbH, Bioperfectus Technologies, Germany), and a real-time polymerase chain reaction (PCR) test was then performed (COVID-19 OneStep RT-PCR kit, Pishtazteb, Iran) using a Rotor-Gene Q Thermocycler (QIAGEN).

Table 1.

-	Total Cases (n)	Sample Size
COVID-19 Wave		n	%
Third Wave
22 Sep. 2020- 21 Oct. 2020	7182	717	10.0
22 Oct. 2020- 20 Nov. 2020	12819	1278	10.0
20 Nov. 2020- 20 Dec. 2020	7354	735	10.0
Total	27355	2730	10.0
Fourth Wave
21 Mar. 2021- 20 Apr. 2021	4955	366	7.4
21 Apr. 2021- 21 May 2021	5541	607	11.0
Total	10496	973	9.3
Fifth Wave
22 Jun. 2021- 22 Jul. 2021	7030	499	7.1
23 Jul. 2021- 22 Aug. 2021)	16504	1255	7.6
23 Aug. 2021- 22 Sep. 2021	11883	498	4.2
Total	35417	2252	6.4
Sixth Wave*
21 Jan. 2022- 19 Feb. 2022	6353	737	11.6
20 Feb. 2022- 20 Mar. 2022	5641	359	6.4
Total	11994	1096	9.1
All periods	85262	7051	8.3

*The period after the widespread implication of COVID-19 vaccines in Iran

3. RESULTS

Among the 7051 individuals, 4411 (62.6%) were male, and 2640 (37.4%) were female. The mean age of the study population was 37.8 ± 14.2 years (range: 1-96 years). The majority of participants (62.0%, 3856/6224) reported at least one COVID-19-associated clinical symptom, and 31.7% (1876/5910) reported a family history of such symptoms. As Table 2 shows, the predominant symptom was cough (42.3%, 2465/5827), while anosmia or ageusia (20.4%) was the least prevalent. The most pervasive chronic comorbidities were cardiovascular diseases (4.4%, 236/5379), followed by hypertension (4.2%, 236/5379), kidney disorders (3.6%, 194/5342), and respiratory diseases (3.5%, 188/5358).

SARS-CoV-2 infection was documented among 3142 individuals (44.6%, 95% CI: 43.3%-45.8%). A marginal disparity in infection rates was observed between males and females (46.4% vs. 41.5%; p < 0.001). Moreover, individuals who tested positive for SARS-CoV-2 using a PCR test were slightly younger than those without infection (37.4 ± 13.8 vs. 38.2 ± 14.5 years, p = 0.029). The infection rates in the third, fourth, fifth, and sixth disease waves were calculated as 45.5% (95% CI: 43.7%-47.4%), 47.7% (95% CI: 44.5%-50.9%), 48.9% (95% CI: 46.9%-51.0%), and 30.4% (95% CI: 27.7%-33.2%), respectively (p < 0.001). Expectedly, the prevalence of COVID-19 was found to be significantly higher in symptomatic individuals compared to those without any clinical presentation (55.5% and 28.2%, respectively, p < 0.001). Besides, individuals diagnosed with COVID-19 were more likely to report a family history of disease-associated symptoms than those with a negative PCR result (39.1% and 25.9%, respectively, p < 0.001). As Table 2 indicates, the prevalence of all six clinical symptoms among individuals infected with SARS-CoV-2 was found to be 1.5–2 times higher compared to uninfected individuals (P < 0.001). Furthermore, individuals with confirmed COVID-19 had substantially higher rates of kidney and respiratory diseases than those without the infection (4.3% vs. 3.1%, P = 0.029 and 4.4%

vs. 2.8%, P = 0.003, respectively). Based on binary logistic regression analysis, participants’ sex, the presence of all six COVID-19 clinical symptoms except sore throat, a family history of SARS-CoV-2 symptoms, and the epidemic period were found to be associated with a higher probability of a positive result in a viral PCR test (Table 3). Among the 6224 people who responded to questions regarding SARS-CoV-2-related symptoms, the infection rates exhibited an increase from 46.6% in the third wave to 49.1% in the fifth wave, followed by a subsequent decrease to 31.0% in the sixth wave (p < 0.001). The pairwise comparison revealed that the percentages of cases of COVID-19 in the third to fifth waves are not significantly different; however, the percentage in the sixth wave is significantly different from those in the other categories (Fig. 2A). Among the clinically symptomatic subgroup, however, the sixth wave demonstrated the highest frequency of confirmed cases (65.0%) (p < 0.001). On the other hand, the highest (44.2%) and lowest (7.4%) rates of infection among the clinically symptomless subgroup were detected in the fifth and sixth waves, respectively (p < 0.001). The pairwise comparisons demonstrated that the percentages observed in the fourth and fifth waves differed significantly from those in the third and sixth waves, yet did not differ from each other (Figs. 2B and 2C).

The rate of SARS-COV-2 infection decreased considerably from 47.2% (95% CI: 45.9%-48.5%) in the pre-vaccination period (the third to fifth waves) to 30.4% (95% CI: 27.7%-33.2%) in the post-vaccination period (the sixth wave, p < 0.001). Furthermore, the percentages of symptomatic participants differ significantly between the two time periods (65.7% vs. 41.0%; p < 0.001). The incidence of a family history of COVID-19 symptoms in the first period (33.8%) was found to be considerably higher than in the subsequent period (20.2%; p < 0.001).

Among the clinically symptomless subgroup, the frequency of confirmed COVID-19 cases in the post-vaccination period showed approximately a five-fold reduction compared to the pre-vaccination period (7.4% vs. 34.5%, p < 0.001). Conversely, within the symptomatic subgroup, the infection rate increased substantially from 54.4% in the first period to 65.0% in the second period (p < 0.001, Fig. 3). As Table (4) shows, during both the pre- and post-vaccination periods, the most prevalent clinical manifestations among the total participants were cough (43.6% and 35.2%, respectively) and sore throat (37.8% and 34.8%, respectively). Among the group without COVID-19, the rates of all assessed symptoms, including fever, chills, sore throat, cough, dyspnea, and loss of taste or smell, were significantly lower in the post-vaccination period than in the earlier period (all p < 0.001). In contrast, among individuals diagnosed with COVID-19, the frequency of fever, chill, cough, sore throat, and dyspnea demonstrated a 1.3–1.8-fold increase during the post-vaccination period (all p < 0.001). The investigation revealed no statistically significant differences in the incidence of taste or smell loss between the two study periods (p = 0.879).

4. DISCUSSIONS

The present study evaluated the COVID-19-related clinical characteristics of the population referred to a medical diagnosis center in northeast Iran during the different epidemic periods of the disease. The prevalence of COVID-19 was documented among 44.6% of the total participants; this figure increased from 45.5% in the third wave to 48.9% in the fifth, followed by a precipitous decline to 30.4% during the sixth wave.

Since its emergence, COVID-19 has undergone genetic evolution, resulting in numerous variants with distinct attributes from the original strain [14]. Different viral variants, such as the Alpha, Beta, Gamma, and Delta variants, have garnered attention due to their increased transmissibility and potential to evade immune responses [15]. These variants have been associated with a range of symptoms and severity levels. The Alpha variant was first identified in September 2020 in the United Kingdom. Patients infected with this variant presented a higher prevalence of symptoms, including cough, sore throat, myalgia, and fatigue [16].

The Delta variant, initially reported in India in October 2020, has demonstrated a doubling in its contagiousness compared to previous variants and has been linked to an increased risk of severe disease outcomes. According to a study by Mahase et al., the Delta variant is associated with a 60% higher hospitalization rate compared to previous variants [17]. Moreover, the probability of admission to the Intensive Care Unit and the fatality risk associated with the Delta variant have been reported to be higher than those of previous variants [19].

Our finding of marginally younger age among SARS-CoV-2-positive individuals aligns with the prioritized vaccination of elderly/ chronically ill groups, thereby shifting infections toward younger, less vaccinated individuals. The Delta variant exhibited clinical symptoms similar to those of previous variants (19). However, this assertion is not universally accepted, as evidenced by the findings of some studies. Torjesen et al. reported that individuals infected with the Delta variant have a lower chance of experiencing a loss of taste or smell [20].

The subsequent variant of concern was Omicron, first described in late November 2021 (18). In comparison to the Delta variant, the Omicron variant has been observed to affect the upper respiratory tract predominantly. It has been reported to be more transmissible, accompanied by a lower hospital admission rate [21, 22]. The present study examined the various COVID-19 epidemic periods in the country driven by the essential variants, including Alpha, Delta, and Omicron variants [23].

Furthermore, vaccination efforts against COVID-19 have played a significant role in determining the clinical manifestation of the disease over time. A body of research has demonstrated that immunization reduces the severity of illness and hospitalization rates among vaccinated individuals. In Iran, the immunization process against COVID-19 began in March 2021, initially targeting medical staff and subsequently prioritizing elderly individuals and those with underlying chronic diseases or disabilities. In the summer of 2021, there was a substantial increase in vaccination coverage coinciding with the peak of the fifth wave, and it was expanded to include all populations in the country [12].

The findings of the present study revealed that the prevalence rates of all related clinical symptoms among the COVID-19-negative group were higher in the period preceding the administration of the vaccine, which was caused by the alpha or delta variants of the virus. On the other hand, the presentation of febrile infections accompanied by chills and respiratory symptoms among people with a positive SARS-CoV-2 PCR test result became significantly more prevalent following the implementation of the country's vaccination program against the virus, predominantly attributable to the Omicron variant. Similarly, Li et al. conducted a comparative study of 384 Omicron cases and 103 Delta cases from China, reporting that these variants exhibited distinct clinical presentation [24]. Their report indicated that people infected with the Omicron variant are more likely to experience a fever. This finding aligns with our study, which showed a heightened frequency of fever among individuals with confirmed COVID-19 during the sixth wave, attributed to the Omicron variant.

Our findings are consistent with those reported by Yang et al., who noted that individuals infected with the Omicron variant were more likely to be asymptomatic or develop sore throats, and less likely to experience anosmia or ageusia compared to those infected with the Beta and Delta variants [25]. While our study found similar results regarding sore throat, we did not find a significant difference in the rate of loss of taste or smell between individuals infected during the pre-Omicron and Omicron waves. Bouzid et al. compared the clinical characteristics exhibited by 818 individuals infected with the Delta variant and 898 infected with the Omicron variant, as referred to in Paris emergency departments [26]. Contrary to the findings of our study, they reported that patients infected with the Delta variant were more prone to manifest symptoms such as fever, cough, and dyspnea, while exhibiting a reduced likelihood of experiencing chills.

CONCLUSION

The present study demonstrated distinct shifts in COVID-19 clinical presentations across epidemic waves in Iran. The Omicron-dominated sixth wave, emerging amid high vaccine coverage, showed a 36% reduction in overall cases but a 1.3–1.8-fold increase in respiratory symptoms (e.g., cough, dyspnea) among confirmed infections compared to pre-vaccination periods. This suggests that vaccine-mediated attenuation of severity is accompanied by persistent transmissibility of newer variants. These findings highlight the need for: (1) variant-adapted boosters to address evolving symptomatology, (2) public education on changing symptom profiles (e.g., prioritizing sore throat/dyspnea testing), and (3) longitudinal surveillance to monitor post-acute sequelae in vaccinated populations. Future research should integrate viral sequencing and individual vaccination records to disentangle the interactions between immunity and variants.

LIMITATIONS

The present study has its limitations. First, individual vaccination status and viral variant data (e.g., genomic sequencing) were unavailable, precluding direct analysis of vaccine efficacy or variant-specific symptom profiles. Second, self-reported symptoms and comorbidities may introduce recall bias. Third, the single-center design limits generalizability to regions with differing demographics or vaccination timelines. Fourth, asymptomatic cases may be underrepresented due to testing biases during later waves.

REFERENCES