The current COVID-19 pandemic has given rise to a lot of speculation as to whether people, once infected with the virus, will be immune to it, and if so, for how long. A recent study published on the preprint server medRxiv* in October 2020 shows that among healthcare workers, and especially among first responders, less than 1% and just over 5%, respectively, had antibodies to the virus.
Since the outbreak of the pandemic, much effort has gone into reducing the rate of spread of the virus. The high infectivity and the sizable proportion of asymptomatic infections have led to tremendous mental stress among frontline medical and other staff. It is estimated that about half of infected cases are asymptomatic for 14 days or more, which could mean the actual number of infections is far greater, up to ten times as high, as that reported.
While many teams of researchers are working to develop a COVID-19 vaccine, the vaccination strategy is still being worked out. Many favor a targeted approach since healthcare workers (HCWs) are at higher risk for transmission within healthcare facilities.
Wide Variation in Seroprevalence
Some surveillance studies on the extent of seroprevalence have shown that there is a wide variation with infection zone, sample size, and methodology, from as high as 57% in Bergamo (Italy) to just ~3% in Santa Clara County, California. This remains true even with a more limited population of healthcare workers, from ~90% in Wuhan to ~3% in Denmark.
It is essential to determine antibody prevalence in this segment of the population, using validated assays and longitudinal sampling, in order to arrive at risk stratifications, and reduce the viral spread in a variety of healthcare settings, as well as shape a strategy for future vaccine delivery.
Low Seroprevalence in HCWs
The current study reports one such study on the seroprevalence among HCWs in a single hospital in California, with follow up at 8 weeks.
The investigators also tested a smaller sample of first responders in the same county, in addition to using the estimated seroprevalence obtained from community physician orders for purposes of comparison. The study was carried out between May and August 2020.
The sample included 3,400 subjects and ~2,700 subjects in the first and second rounds, respectively. Among the first responders, the numbers were 226 and 92 for the first and second rounds, respectively.
The observed seroprevalence among HCWs in the first round was ~1%, increasing to ~2.8% at the 8-week follow-up. The latter included 28 persistently positive subjects out of the 32 in the first round, with another 43 subjects being identified as seropositive. Adding in the four positive cases among HCWs who did not return for the final blood draw brought the adjusted seroprevalence at 8 weeks to 2.7%.
Just under half the HCWs who had a PCR-positive test status for COVID-19 turned out to be seronegative at the 8-week follow-up point. This agrees with some recent studies that have shown that IgG antibody titers decline rapidly within about 3 months in patients with mild infection. However, a seronegative result should not be interpreted as a lack of immunity, and these results need further study in larger groups.
All of these patients had a history of mild illness. The positive subjects were all significantly younger and more symptomatic than those who were seronegative. The observed negative cases differed from the positive cases by age, race, and symptoms, but not by occupational risk (as HCWs).
The seroprevalence among HCWs is relatively low. The community prevalence at this time was ~3.6%, also a relatively low figure, since it comes from a period when the state was under “shelter at home” orders. Rigorous training and education practices were implemented to allow HCWs to take precautions against infection both in and outside the workplace.
Seroprevalence Among First Responders
Among the first responders, the prevalence was ~5.3%, with an adjusted seroprevalence of 5.8%, with the positives differing from the negatives by symptoms like fever, cough, and anosmia.
At the 8-week follow-up, the observed and adjusted prevalence was ~4.3%, 1 case remaining positive out of the original 12, while 11 were not tested in the second round. There were 3 new cases at this point. When corrected for the 11 who failed to return for the second testing round, the final prevalence among first responders was ~14.6%.
Potential Role of Innate Immunity
Another explanation offered for this observation is that HCWs are more likely to have pre-existing non-specific coronavirus antibodies and T cell mediated immunity because of prior infection by other coronaviruses, than the community at large.
Some studies have shown that family members of patients with confirmed infection are immune in this sense, in up to a third of cases, without having been infected themselves. This has also been observed in blood donors. This could help explain why younger children are less susceptible to SARS-CoV-2 infection, since they are exposed to a wide variety of less pathogenic coronaviruses by their peers in school.
The researchers conclude: “Our findings suggest that the recommended in-patient personal protective equipment is effective in reducing the risk to HCWs.”
Also, the role played by innate immunity may be greater than realized until now, particularly among HCWs. The findings also indicate that HCWs who were seropositive initially continued to be so for 8 weeks at least, and no reinfections were noted, encouraging the prospect of durable immunity.