In this study, we found that vaccination had no significant influence on symptomatic disease, while fever and loss of smell or taste are significantly less likely to occur when a person has previously recovered from COVID-19, irrespective of vaccination. There is also a noticeable trend of participants with no pre-existing immunity experiencing longer symptom duration overall and are more likely to develop pneumonia. Negativization period tends to be the shortest among fully vaccinated and previously infected participants, followed by groups that were either vaccinated or previously infected, suggesting a faster viral clearance in people with vaccine-induced and/or infection-induced immunity. When comparing Ct values of 4 analysed genes as measures of viral load, we found no significant difference between our groups, indicating similar initial viral load across our study population.
Taking into consideration the time period of our study and high incidence of cases at the time, we assume that the dominant SARS-CoV-2 variant was Omicron (B.1.1.529). WHO (World Health Organization) classified it as VOC (variant of concern) on November 26, 2021, while the first confirmed case in Bosnia and Herzegovina was on December 29, 2021 [1,2,6]. Our results regarding clinical manifestations are consistent with previous studies that indicate substantially lower vaccine effectiveness against symptomatic disease caused by the Omicron in comparison to previous variants [7,8]. Even though their effectiveness is reduced, vaccines do provide adequate levels of protection against symptomatic disease in the first few weeks after administration . However, the majority of our vaccinated participants (82.6%) have received their second dose 5 or more months before enrolment in the study, which might additionally influence the lack of symptom variation among 4 observed groups. Studies suggest that the Omicron variant is associated with the ability to evade immunity from prior infection [10-12]. That is also evident from our study, where 50.4% of participants have reported a previous SARS-CoV-2 infection confirmed by RT-qPCR, of which 90% tested positive 5 or more months before our study. Considering that the large majority of participants had COVID-19 or had been vaccinated with 2 doses well before enrolment in the study, we can assume that their immunity waned over time. Furthermore, our previous study demonstrated sustained titers of SARS-CoV-2 IgG antibodies over one year of infection, implicating that specific neutralizing IgGs are crucial for control of virus spreading rather than sustained IgG titers .
Studies showed that the Omicron variant has less impact on the lower respiratory tract, with major clinical manifestations being those of a "mild infection“ [14,15]. This is consistent with our findings, where only 5% of study participants developed pneumonia, and none were hospitalized. It is worth noting that we observed 9.1% of participants with no pre-existing immunity (group 4) developing pneumonia, suggesting that either vaccine-induced or infection-induced immunity do play a role in the prevention of serious disease outcome. To expand on the observed trends regarding group 4, it is notable that they experienced longer overall symptom duration (26.8% had symptoms for ≥7 days), and fever duration (33.3% had fever for more than 3 days). This directly correlates with longer viral clearance, where more than 50% of HCWs from group of participants without any pre-existing immunity tested negative after 8 or more days. However, it is not yet known how long infective virus persists in patients through different stages of infection, but it has been confirmed that successful SARS-CoV-2 cultivation after 8 days from sampling or symptom onset is possible [16-18]. Worth noting is that other participants, regardless of pre-existing immunity, also showcased prolonged viral clearance to some extent. This raises a concern since the latest CDC (Centers for Disease Control and Prevention) recommendation shortened isolation time from 10 to 7 days for HCWs, as long as they don’t have symptoms . Taking into consideration a relatively low rate of vaccination among HCWs from our study (36.9%), and a significant number of HCWs with no pre-existing immunity (31.2%), we should emphasize on protective equipment utilisation in hospital environment. In particular, masks should still be obligatory, regardless of the immune status of HCWs, since Omicron variant is more transmissible when compared to previous variants . This could provide additional protection to HCWs who are at occupational risk of SARS-CoV-2 infection, as well as to patients .
Viral load following Omicron infection does not seem to be related to increased infectiousness, as it was the case with previous VOCs [20,21]. In our study, Ct-values were utilized as surrogate markers for the amount of virus in a specific sample. The relevance of Ct-value threshold as a measure of potential infectivity is still unclear . We observed that there is no significant difference in Ct values of analysed genes between our 4 groups, indicating a consistent initial viral load in participants, irrespective of vaccination or previous SARS-CoV-2 infection. Omicron’s immunity evasion may be associated with unusually large number of mutations in the Spike (S) protein, which is responsible for binding and entry into host cells. Relative to the original strain, Omicron has 37 mutations in the S protein, of which 15 are in the receptor-binding domain, which is the target of many neutralizing antibodies [3,4,22].
Our study showed significant correlation between prior contact with the SARS-CoV-2 and fever onset upon reinfection, irrespective of vaccination status, suggesting that infection-induced immunity could play an important role in the prevention of symptomatic disease. Furthermore, data from the UK shows that loss of taste and smell was less commonly reported at the end of December 2021 (16% / 13%), as compared to the start of December 2021 when Delta variant dominated (44% / 44%) . Our findings coincide with this data, where 14.2% (20/141) of our study participants reported loss of smell or taste. However, we found that previous infection strongly affected the occurrence of this symptom, irrespective of vaccination, suggesting a stronger upper-respiratory mucosal immunity after natural infection. This may be related to the fact that the current COVID-19 vaccines are administered intramuscularly (systemic vaccination), and primarily induce antibodies of the IgG class, and little to no respiratory IgA . On the other hand, IgA are the most effective antibody class on respiratory mucosae, and several studies have found that they possess superior anti-viral properties vs. IgG for SARS-CoV-2 [25-27]. In support of this, hybrid immunity as a result of natural infection and vaccination, could prevent both localized and systemic infection by VOCs, due to significantly larger boost to the neutralizing antibody response compared with 2 doses of vaccine or previous infection alone [28,29]. In conclusion, systemic vaccination of HCWs along with consistent protective equipment utilisation might ensure an overall safer environment in healthcare facilities.
This study has several limitations. Small number of participants resulted in difficulty finding significant relationships from the data. Observing a larger group of HCWs should result in a more realistic distinction regarding our study points. Furthermore, we did not provide comparison between different vaccine types, as there was an overall small percentage of vaccinated participants as a result of low availability of vaccines at the time, and we found no significant correlation between reported vaccine types and any objectives observed in this study (data not shown). Similarly, we only included HCWs who received 2 doses in the study, since there was a very limited number of participants who reported receiving a booster dose, which did not influence our results. Additionally, Omicron variant was only assumed, not identified in each study participant. However, the study period coincides with the highest prevalence of COVID-19 in Bosnia and Herzegovina and the period in which Omicron predominated, after two years of pandemic . Finally, we did not take antibody titers into consideration when establishing previously infected groups, as the data reported was scarce and inconsistent, and in majority was provided by participants who previously recovered from COVID-19 (as confirmed by RT-qPCR).