Coronavirus Disease 2019 (COVID-19), caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), has profoundly impacted global health since its emergence in late 2019 (1–4). The pandemic has led to significant morbidity and mortality worldwide, overwhelming healthcare systems and disrupting economies (5, 6). With millions of confirmed cases and deaths, COVID-19 has highlighted the vulnerability of global health infrastructures to emerging infectious diseases (7, 8). The virus’s rapid spread and high transmissibility have necessitated unprecedented public health measures, including lockdowns, social distancing, and mass vaccination campaigns, to mitigate its impact (9–11).

The pandemic has unfolded in multiple waves, each characterized by different variants of the virus, varying levels of transmissibility, and distinct patterns of morbidity and mortality (12). These waves have placed immense pressure on healthcare resources and have been associated with significant fluctuations in case numbers and healthcare demand (13). The emergence of new variants, such as B.1.617.2 and B.1.1.529, has complicated efforts to control the pandemic, with each variant posing unique challenges in terms of transmissibility, vaccine effectiveness, and disease severity (14).

In Iran, the burden of COVID-19 has been substantial, with the country experiencing several waves of infection that have strained its healthcare system. High rates of infection and mortality have been reported, particularly during the peaks of the pandemic (15). The Iranian healthcare system has faced numerous challenges, including shortages of medical supplies, overwhelmed hospitals, and difficulties in implementing public health measures (16). Despite these challenges, efforts have been made to enhance testing, treatment, and vaccination to control the spread of the virus and reduce its impact on the population.

Despite extensive global research on COVID-19, understanding reinfection, particularly its clinical implications and associated variants, remains limited. Given the high burden of COVID-19 in Iran and the evolving nature of the virus, it is crucial to investigate SARS-CoV-2 reinfection in in this specific demographic. The aim of this study was to identify the rate and the risk factors associated with SARS-CoV-2 reinfection and compare the clinical course of initial infection versus reinfection in readmitted COVID-19 patients in Iran.

The results of our study indicate that among the 0.5% of cases that met the WHO criteria for reinfection, these individuals had milder disease manifestations and a milder clinical course than those with primary infections. It has been suggested that reinfection is more pronounced in the new wave of SARS-CoV-2 variants and that full vaccination, especially booster vaccination, may be effective in preventing reinfection. It is noteworthy that the B.1.1.529 variant was more prevalent among those who had experienced reinfection. These findings contribute significantly to our understanding of virus dynamics and inform ongoing efforts in public health strategies, particularly in the context of emerging variants and vaccine efficacy.

It is crucial to distinguish between a reinfection with the SARS-CoV-2 virus that causes the disease and a return of symptoms. These two conditions can be challenging issues in clinical practice (22). The diagnosis of reinfection with SARS-CoV-2 is complex. Reinfection can sometimes be overreported based on clinical symptoms or radiological findings, and in some cases, PCR tests might be negative despite an active infection (23). In such scenarios, patients might experience a relapse of the original infection, or they could have other diseases with symptoms similar to SARS-CoV-2, such as influenza or respiratory syncytial virus (24). In addition, many individuals experiencing reinfection may not seek medical attention or may only visit outpatient clinics, thus not being captured in hospital-based data (25). The current study assumed a reinfection rate of 0.5%. This result is consistent with previous studies in the current literature. In the meta-analysis of 23 studies in 2023, the reinfection rate ranged from 0.1 to 6.8% (26). In addition, in a recent meta-analysis in 2024, the pooled rate of reinfection in 55 studies was estimated to be 0.94% (27). The observed difference in the reported rate of reinfection may be mainly due to differences in patient selection criteria. In our study, we made a concerted effort to carefully select patients in a manner that minimized the likelihood of false positives while adhering to WHO guidelines.

Our results align with previous studies suggesting that reinfections generally present with less severe outcomes compared to primary infections (28). The observation that cases of reinfection had shorter hospital and ICU stays is consistent with other reports indicating that immune responses from prior infections or vaccinations might reduce the severity of subsequent infections (29). Furthermore, the discrepancy in the predominant SARS-CoV-2 variants between reinfection and primary infection cases highlights the virus’s evolving nature and its impact on disease presentation (30). The lower frequency of full vaccination in reinfected individuals compared to the non-reinfected group also underscores the need for continued research into the effectiveness of current vaccines against emerging variants (31). Although this difference was not statistically significant following PSM, it remains critical to continue evaluating vaccine efficacy, particularly in the context of emerging variants. The effectiveness of vaccines against variants has been shown to diminish over time, necessitating booster doses to maintain protective immunity (32). Our study’s findings are consistent with the literature, which indicates that while vaccines remain effective in preventing severe disease, the evolving nature of SARS-CoV-2 variants calls for regular updates to vaccination protocols and booster recommendations (33).

The distribution of SARS-CoV-2 variants in the reinfection and control groups highlighted a clear trend toward a higher proportion of reinfections during waves dominated by newer variants. Reinfections were less frequent during the B.1.1.7 wave. This trend shifted significantly with the emergence of the B.1.617.2 variant and became even more pronounced during the B.1.1.529 wave. The increasing reinfection rate in later waves is consistent with the immune-evading properties of newer variants such as B.1.1.529, which have been shown to have higher transmissibility and reduced vaccine efficacy (34). Studies have shown that B.1.1.529, with its numerous spike protein mutations, is more adept at escaping both natural immunity and vaccine-induced immunity, leading to increased reinfection rates (35). Our analysis also demonstrated that individuals who received a vaccine booster were significantly less likely to experience reinfection, even during the B.1.1.529 wave, highlighting the positive effect of booster doses in enhancing protection against immune-evading variants. This is further supported by studies showing that booster doses can restore vaccine efficacy against variants such as B.1.1.529 (36).

The strength of our study lies in its large, representative sample of over 30,000 confirmed SARS-CoV-2 infection cases from five major hospitals in different regions of Iran, ensuring a robust analysis of reinfection patterns and outcomes. By adhering to WHO criteria for reinfection and focusing on hospitalized patients with confirmed PCR results, we ensured high diagnostic accuracy. The study’s findings of greater susceptibility to reinfection during the B.1.1.529 wave and the protective effect of booster doses are consistent with global research and underscore the importance of booster vaccination against immune-evading variants. In addition, the inclusion of regional data highlights the impact of healthcare disparities in Iran, where differences in access to resources may influence hospitalization and outcomes.

It is important to acknowledge the limitations of our study, despite its robust design. The first limitation is the reliance on PCR testing as the sole method for confirming SARS-CoV-2 infection, which may not capture all cases, particularly those with low viral loads where PCR sensitivity might be reduced. This could potentially lead to an underestimation of reinfection rates. However, by focusing on positive PCR results, we ensured diagnostic certainty and accuracy in identifying true reinfection cases. Another significant challenge encountered in this study was the limited access to diagnostic kits for SARS-CoV-2 variant identification in Iran. Due to sanctions, these kits were frequently unavailable or in short supply, which constrained our capacity to accurately identify specific variants. Additionally, many of the reinfection cases occurred after the introduction of variant-specific diagnostic kits, whereas a large portion of the control cases were from a period when new variants had not yet been identified. Consequently, variant data, particularly in the control group, may have been underrepresented. To address this imbalance and ensure a robust comparison between the two groups, we applied PSM. Another limitation is the lack of detailed data on treatment scenarios. Although there are established guidelines for COVID-19 treatment, the treatment protocols changed over the pandemic time (37). In addition, the administration of management strategies varied depending on the availability of pharmaceuticals and the limitation of resources in different cities and medical centers. The last limitation of our study is the lack of extended follow-up data. Although reinfections generally present with milder symptoms, they may be associated with severe long-term complications such as stroke, myocardial infarction, deep vein thrombosis, or pulmonary embolism (38).

This study recommends prioritizing revaccination campaigns to improve protection against reinfection with SARS-CoV-2, especially in the face of emerging immune-evading variants such as B.1.1.529. Public health strategies should be regularly updated to reflect the evolving nature of the virus, and efforts must be made to improve access to variant-specific diagnostic tools in resource-limited settings. In addition, future research should focus on longer-term follow-up to assess potential complications of reinfection, such as cardiovascular events, to provide a more comprehensive understanding of the long-term effects of SARS-CoV-2.

SARS-CoV-2 reinfection generally exhibited milder symptoms and shorter hospital stays than primary infections. Novel SARS-CoV-2 variants were more common among reinfected individuals. Although vaccination can help prevent reinfection, the complex relationship between vaccination and reinfection highlights the need for further research. Future studies are needed to assess the long-term complications of SARS-CoV-2 reinfection.