Introduction
Since November 2019, the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has infected more than 134 million people worldwide and has caused more than 2 million deaths [1]. No curative drug or specific treatment is known to have considerable efficacy against this infection; but, currently over 200 candidate vaccines have been acknowledged by the WHO [2-4].
Our experience with other respiratory infections, such as those caused by influenza and seasonal respiratory infections caused by coronavirus lead us to believe that the natural immunity after an episode of infection with this virus may not cause long-time immunity [5]. Some studies have shown a substantial wane in antibody levels within a few months after remission [6, 7]; and the studies that report re-infection further question the efficacy and longevity of the acquired immunity after infection with this virus. Several cases of suspected and proven instances of re-infection with SARS-CoV-2 have been reported in immunocompetent patients in different age groups, in patients with different levels of antibody response [8-10].
The duration of sustained antibody response after an episode of infection is generally a representative of the immune protection that can be achieved by vaccination against the same virus [11]. Waning of the IgG antibodies that are naturally produced in a patient after infection with SARS-CoV-2, undermines the supposed efficacy of vaccination; especially since cases of symptomatic re-infection with replication-competent virus have been reported within the first 6 months after the initial infection [12, 13].
To this date, thousands of distinct variants of SARS-CoV-2 have been identified with over 400 variants in the spike protein, which is presumably the binding cite of neutralizing antibodies [14]. Besides, in many cases of re-infection, genetic sequencing has revealed a different clade of the virus to be the causative pathogen [15-18]. We can assume—as is the case with infections caused by rhinoviruses and influenza—that the protective activity of antibodies is limited to each specific subtype of the virus, this can possibly explain why re-infection can occur in presence of detectable levels of IgG [19, 20].
In this study we investigated the rate of symptomatic and asymptomatic re-positivity with SARS-CoV-2 in recovering patients for up to four months after the initial diagnosis of Coronavirus Disease 2019 (COVID-19); and find possible risk factors that are associated with re-infection.