Between March 15th and June 21st, 2020, the Spanish government imposed a national lockdown in an attempt to control the extent of the COVID-19 due to SARS-CoV-2 coronavirus outbreak within the national territory 14. Mobility restriction was among the exceptional measures taken, limited to the inhabiting city first and surrounding province in a second step. Only those people considered as essential personnel could move to attend their working places such as firefighters, civil servants preserving public safety and of course, healthcare workers (HCWs). Those people showing mild to moderate symptoms were asked to self-isolate at home, keeping clinical facilities for severe cases. The same preventive measures were asked for those asymptomatic population known to have been in contact with people showing COVID-19 compatible symptoms. Keeping in mind this situation, a nationwide, population-based seroepidemiological study performed between April 27th and May 11st 2020, have shown that the seroprevalence against the SARS-CoV-2 in Spain was around five percent 11. Interestingly, this seroprevalence was not uniform along the country, with areas displaying higher IgG positive rates such as Madrid province, with up to 11.5% of seroprevalence 11.
Under this scenario, we performed the current study, analyzing the IgG seroprevalence against SARS-CoV-2 in individuals that suffered COVID-19 compatible symptoms at least two weeks before performing the test, that knew to have been in contact with a COVID-19 diagnosed person or that belonged to an essential collective that kept working during the lockdown. Following this criteria, in a large cohort of 449 individuals, we report here an IgG seroprevalence of 33.69%, much higher than the global Spanish rate and even than the prevalence shown in the Madrid province, where our study has been performed considering mobility restrictions.
The high IgG+ rate observed in our cohort is surely conditioned by the inclusion criteria. At the same time, this is the strength of the data provided in here. Our results illustrate how the seroprevalence rises in a population presenting symptoms and at high-risk exposure, with no further bias such as occupation. To the best of our knowledge, no studies gather such a population without discriminating by other factors. Therefore, the features included in our cohort are not dissected in population-based wide studies such as those performed in Spain 11 or Switzerland 16, nor in published studies addressing the seroprevalence against SARS-CoV-2 specifically in HCWs 12,13,17–20.
In this sense, our data confers to HCWs an IgG seroprevalence of 25.58% (95% IC: 13.52 – 41.17). Other studies focused in these professionals reveals rates ranging from 2% 18, 7.5% 17,20, 10% 11–13 to nearly 20% 19,21. Our results are in the upper range and backed by recent data from a large cohort of HCWs in a secondary teaching hospital in Madrid province 22. These results are consistent with a close to two-fold higher risk for health-care workers indicated in the Spanish national seroprevalence study, as the reference population for our cohort would be the IgG+ prevalence in Madrid province, being 11.5% 11. Similarly, in two independent seroprevalence studies performed in Barcelona province, also a nearly two-fold higher risk was described for HCWs, ranging between 9.3% 12 and 10.3% 13 compared with 7% in the general province population 11. Also in Germany, a population-based seroprevalence study denoted a 0.94% of IgG seroprevalence in the North Rhine-Westphalia region 23, while a 1.6% was reported among healthcare workers of the University Hospital Essen located in the same region 18.
Interestingly, no significant differences in IgG seroprevalence were found between HCWs in our cohort and other high-risk exposure professions including firefighters or public safety personnel such as police. These data suggest that despite much attention is being payed to people working in front-line clinical assistance, other public employees are similarly high-risk exposure and they should share personal protection and training with HCWs to protect themselves from the coronavirus infection. Indeed, all these key professionals show the same IgG seroprevalence than people that manifested COVID-19 compatible symptoms or that established a contact with a confirmed case, as these are the other two inclusion criteria for our cohort.
A relevant factor to explain the high IgG seroprevalence against SARS- CoV-2 in the studied cohort could be the controlled period of time spent between the end of the symptoms and the test. One of the abovementioned studies was performed among HCWs exposed to COVID-19 patients; these HCWs were under quarantine for 14 days, time when their seroprevalence was analyzed 21. Interestingly, the seroprevalence was in the highest range among the studies performed in HCWs (17.14%) 21. It could be suggested that this high rate was due to the exposition to confirmed cases, but leaving time enough to develop a robust humoral IgG response 24. In fact, plasma from COVID-19 convalescent patients to be used as potential treatment against the infection, show significantly higher anti-SARS-CoV-2 titers after more than 42 days from symptom onset to plasma donation 25. For those symptomatic subjects of our cohort, at least 14 days after the end of symptoms were required before performing the test with an average time gap of more than 40 days. Therefore, timing from the suspected contagious contact with the virus should be also considered and detailed, in an attempt to achieve a better interpretation of the seroprevalence data from diverse cohorts.
An important value of our data is to provide information about anti-SARS- CoV-2 IgG seroprevalence in a cohort where most of the subjects manifested COVID-19 compatible symptoms, but none of them required hospitalization. Early observations in hospitalized patients reported IgG+ rates as high as 80% 26,27 to even 100% in longitudinal studies 28. These data seeded the hope of reaching a protective herd immunity 29. However, wide nationwide studies are reporting low seroprevalence rates 11,16,30, which are promoting pessimistic perspectives between experts regarding the achievement of such herd immunity 31. Noteworthy, the seroprevalence observed in our cohort indicates that the predominance of IgGs against SARS-CoV-2 may be quite heterogeneous between populations, and heterogeneity affects massively herd immunity, modulating the percentage of infected people required to reach it 10. Therefore, seroprevalence studies in controlled settings as the one performed here could be really helpful to understand population heterogeneity related to herd immunity.
Despite the test we used for this study detected both IgM and IgG against SARS-CoV-2, we decided to perform the seroprevalence study based exclusively on IgG. The main reason was the inclusion criteria of having suffered COVID-19 compatible symptoms at least, 14 days before testing, to fulfill the quarantine period established by the sanitary authorities. In line with this rationale, only 2% 28of the individuals included in our cohort was tested IgM+. Indeed, IgG detection is the base for most if not all population-based seroprevalence studies 11,16,30. However, it does not preclude the usefulness of IgM determination for early infections together with PCR as gold standard, showing a great concordance with IgG in the long term 27. In this sense, and in agreement with other works 11, we observed a great correlation between IgG+ seroprevalence in those individuals that self-reported a previous positive test by PCR. However, this correlation is not 100% and in fact, this result is a constant in essentially of the analyzed studies 28,32. False positive or negative results could explain to some extent this supposed discrepancy. However, the generation of T-cell based cellular immunity could also be an underlying mechanism 33.
It has been described the generation of immunogenic CD4+ and CD8+ responses against SARS-CoV-2 antigens that correlate with the presence of specific antibodies against the virus 34,35. However, up to 35% of unexposed donors (without detectable anti-SARS-CoV-2 specific antibodies) showed SARS- CoV-2-reactive CD4+ T cells, while 83% of antibody-bearing patients displayed comparable cellular responses 36. Interestingly, the SARS-CoV-2-responsive CD4+ T cells from unexposed donors also responded to similar epitopes present in endemic coronaviruses 229E and OC43 36. Therefore, cross-reactive T cells might be the base for this cellular activation in the absence of humoral responses, although the dynamics of this cross-reactivity is not understood yet. As greater T cell expansion is observed in mild versus severe COVID-19 37, it is tempting to speculate that T cell immune responses are predominant in mild SARS-CoV-2 infections.
In this sense, cellular immunity mediated by T lymphocytes could also explain the presence of uninfected household members. Despite that IgG seroprevalence rises statistically among those subjects in contact with a confirmed case of SARS-CoV-2 infection 11, nearly half of individuals of our cohort was seronegative for the coronavirus despite sharing home with a COVID-19 positive person. Interestingly, only mild symptoms were reported in a cohort of non-hospitalized household contacts with SARS-CoV-2 infection 38, further supporting the notion that cellular immunity triggered by mild infections could underlie the lack of seroconversion. Large studies comparing mild versus severe COVID-19 cases, analyzing both cellular and humoral responses are required to fulfill this gap in our knowledge.
An intriguing data arisen from our cohort is that smokers show a reduced IgG seroprevalence against SARS-CoV-2. There could be many plausible explanations for this effect that nowadays are just speculations. It includes a direct toxic effect of smoking against the virus, impact on the hACE2 expression as the viral entry gate or a pre-inflammatory status of the pulmonary tract that helps to reduce the initial virus burden or even reduction in the production of antibodies. In any case, a large observational study enrolling nearly 150,000 participants supports that the SARS-CoV-2 infection is reduced by half among current smokers 39.
As one of the criteria to be included in our study is the presence of compatible symptoms, it allows to study the relationship between their development and the seroprevalence in the studied population. Notably, there is a nice positive correlation between the number of presented symptoms and the anti-SARS-CoV-2 IgG seroprevalence. This would suggest that the strength of the physical manifestations due to COVID-19 reinforces the development of antibodies against the virus 40, but also that many isolated symptoms are not specific enough for the development of COVID-19, but this lack of specificity is attenuated by the combination of non-specific manifestations.
In any case, it is interesting to note that the correlation of some symptoms with the anti-SARS-CoV-2 IgG seroprevalence is much stronger than others. This supports the notion that COVID-19 is manifested with some sort of symptomatic specificity, that might help to identify and isolate infected individuals in an potential scenario of shortage of diagnostic tests. In this line, the development of ageusia/anosmia, pneumonia and cutaneous manifestations highly correlate with IgG seroprevalence, ranging between almost 80% and more than 60% when any of these symptoms appears. This data indicate that these three clinical indicators could be considered as clear determinants for the diagnosis of COVID-19 41,42.
In sharp contrast, when considering the symptoms developed among IgG+ participants in the studied cohort, it is notorious that pneumonia, cutaneous manifestations and even dyspnea are among the less frequent presented symptoms. This fact signifies that depending on the intrinsic features of the participants, some symptoms are more or less indicative of COVID-19. The studied cohort in particular, gather a population of high-risk exposure and symptomatic individuals that did not required hospitalization and therefore, it was unlikely to have a clinical diagnosis of pneumonia, dyspnea or even cutaneous manifestations that require skilled medical personnel to detect them. Consequently, it could be proposed the presence of ageusia/anosmia plus fever as the most accurate indicative symptoms for COVID-19 in the general population without the need of medical assistance 43. The appearance of both signs could motivate a self-isolation and to contact health authorities.
In summary, the data presented in this work illustrate the IgG seroprevalence against SARS-CoV-2 in a symptomatic and high-risk exposure large population. This seroprevalence was nearly two-fold higher than what has been described in a population-based, nationwide study for the same geographic area. This relative high seroprevalence is shared between different front-line public professionals such as healthcare workers, firefighters and public safety personnel. Importantly, the seroprevalence among these professionals is comparable to participants with other occupations, either showing symptoms or inhabiting with a COVID-19 confirmed case. The number of symptoms reported positively correlate with IgG seroprevalence. Among these symptoms, ageusia/anosmia, pneumonia and cutaneous manifestations are the three symptoms correlating with higher rates of IgG seroprevalence. However, the most frequent symptoms among IgG+ participants are fever, ageusia/anosmia and asthenia. Therefore, heterogeneity among populations should be considered when defining seroprevalence and key diagnostic symptoms.