Humoral response
Both insufficient and overactive immune responses have been reported in COVID-19 patients [45]. Production of protective cytokines and IFN-gamma, which is mediated by the CD4+ and CD8+ cells, plays an important role in containing and resolution of the infection [46]. The dynamics of the antibody response in COVID-19 patients is not completely known; and different studies have reported different rates of seroconversion. Zhao et al [47] and Liu et al [48] reported seroconversion in all infected patients respectively by 39 and 14 days after the onset of infection. Liu et al also reported that by the 60th day IgM antibodies were undetectable in about one-third of the patients and the IgG titers had decreased substantially [48]. Based on another study, although recently discharged patients have a high level of humoral immunity against the virus, antibodies start to decreases within 2 to 3 months after the infection [13]. In another study the seroconversion rate for IgG, IgM and IgA was ~90% and most patients seroreverted within 75 days; with IgG levels remaining detectable over 90 days after the symptom onset in more than 99% of patients [49]. Multiple studies have also concluded that the humoral immunity against this virus could be short-lived [50]. Contrasting these studies, our results showed that 94% of patients were positive for neutralizing antibodies (IgG) 120 days after the onset of symptoms; which is in line with the results of an Icelandic population study that reported a 91% seropositivity four months after the initial diagnosis of COVID-19 [6]. In evaluation of the results of these studies, we should take into account the natural process of the humoral response. In case of many other viral infections—where seroconversion is sustained as seromaintenance and immunity—we see a temporary decrease of antibody levels during the first few months of infection/inoculation [51], and since the emergence of COVID-19 is recent, we cannot judge the humoral response to this virus in long term and a rebound increase in antibody levels can be expected [50].
In our study, the four patients who had a positive result in RT-PCR screening 120 days after the initial diagnosis of COVID-19, were also positive for neutralizing antibodies; and although they theoretically may have prevented a severe episode of re-infection and caused a lack of any symptoms in one RT-PCR-positive patient, we cannot know for sure if those levels are high enough to be completely protective [21]. In a similar study Zhang et.al reported re-infection in 6 recovered patients that was caused by viruses from lineages different from the first infection. All these patients had varied levels of antibodies and they concluded that presence and even maintenance of the humoral response cannot rule out the possibility of re-infection [52]. We also believe that the two patients who did not have sufficient levels of IgG (<1.1 g/L), have been protected from an episode of re-infection by a strong cellular immune response, even within an epidemic situation.
Genetic sequencing can rule out a false positive in RT-PCR testing and determine if the infection is caused by a different subclass of the virus. In cases of re-infection with a different clade of the virus, even protective levels of IgG may not be effective [21]. We hypothesize that high levels of neutralizing antibodies do not make the diagnosis of re-infection unlikely, unless there is genetic proof that the positive RT-PCR results are related to the same strain of the virus from the first episode; in which case re-activation/relapse would be a more likely diagnosis.