Workplace contact was defined as a firefighter who was within 6 feet of an infected person (laboratory-confirmed or probable COVID-19 patients) for at least 15 min; the definition is consistent with the US CDC guidelines. SARS-CoV-2 antibody test administration A rapid immunoglobulin (Ig)M-IgG combined point-of-care (POC) lateral flow immunoassay (BioMedomics, Morrisville, New Carolina, USA) was used for assessment of SARS-CoV-2 antibodies in participating firefighters/paramedics. 13 The sensi- tivity and specificity of the COVID-19 antibody assay were estimated to be 88.66% and 90.63%, respectively, based on the results for 397 infected cases and 128 non-SARS-CoV-2 infection patients in Wuhan, China. 14 The positive predictive value (PPV) of the test is estimated to be 33.2% and the negative predictive value is 99.3%. On testing day, on-duty and off-duty firefighters/ paramedics drove through a structured drive-through lane at the fire station/training facility. Firefighters/paramedics rolled down their window, were approached by the F-TRACE team gowned in PPE for an initial index finger swab with rubbing alcohol, followed by a quick lancet finger puncture to allow for two drops of blood to be placed in the cassette sample well. F-TRACE team members added two drops of buffer reagent to the cassette sample well and waited 10 min for the test to complete prior to reading the results. Data analysis We calculated descriptive statistics for continuous variables, expressed as means with its SD, and for categorical variables, expressed as frequency and percent of the sample. We examined the main outcome of testing positive (combined IgG only, IgM only and IgG/IgM) by sociodemographic and work character- istics, by COVID-19 contacts and COVID-19 symptoms. For categorical data, we conducted Fisher’s exact test to compare groups. Student’s t-test was used to compare the mean days of symptom onset, firefighter tenure, time in fire department, average number of COVID-19 case contacts, average time spent with COVID-19 cases and the number of PPE items used with COVID-19 case between firefighter who tested positive versus negative. P values less than 0.05 were considered statistically significant. We performed all data management and statistical analyses using SPSS V.26 for Windows (IBM). RESULTS Among the 203 firefighters/paramedics that participated in the F-TRACE project, 18 firefighters/paramedics (8.9%) tested positive for SARS-CoV-2 antibodies, of which 8 firefighters/ paramedics (3.9%) were IgG positive only, 8 (3.9%) were IgM positive only and 2 (0.1%) were IgG/IgM positive (table 1). None of the antibody positive firefighters/paramedics reported receipt of the annual influenza vaccine compared with fire- fighters/paramedics who tested negative for SARS-CoV-2 anti- bodies (0.0% vs 21.0%; p=0.027). Although not significant, the proportion of firefighters/paramedics who reported symptoms in the 2 weeks prior to antibody testing was higher for those who tested antibody positive compared with firefighters/paramedics who were antibody negative (22.2% vs 7.7%; p=0.064). The average number of COVID-19 case contacts was significantly higher (13.3±4.8 case contacts vs 7.31±4.8 contacts; p=0.022) among firefighters/paramedics who were SARS-CoV-2 antibody positive compared with firefighters who tested negative for antibodies.
DISCUSSION As a component of an overall medical surveillance programme, we found variation in the seroprevalence of SARS-CoV-2 antibodies among frontline firefighters/para- medics of a moderately sized US fire department. Approxi- mately 4% of the participating firefighters/paramedics tested positive for either IgM or IgG/IgM SARS-CoV-2 antibodies, indicating recent infection from the time of immunoassay antibody testing. These findings provided timely and useful information on decision to quarantine and further evaluation through reflex RT-PCR nasal swabs. Nonetheless, caution at interpreting the results of the antibody testing is warranted. When the prevalence of COVID-19 is based on serology testing (ie, all antibodies including IgM only, IgG only and combined IgM/IgG) and the prevalence of COVID-19 is esti- mated to be low (eg, 5% within the workforce), the risk of false positives can be elevated. For example, if the COVID-19 serological test has 90% specificity, we estimate that its PPV will be 32.1%, thus nearly 70% of positive results will likely be false. At this same disease prevalence (~5% of the work- force), a test with 95% specificity will lead to a 50% chance that a positive result is incorrect. Similarly, it is possible that a positive result on COVID-19 antibody serology test can be due to cross-reactivity with other viruses. Different assays use antigens from different parts of SARS-CoV-2, and some combine IgM and IgG, therefore different levels of cross- reactivity with other coronavirus antibodies are possible. 15 At the time of this pilot study (April 2020), the BioMedomics COVID-19 assay was used by our team under the Emer- gency Use Authorisation (EUA) authority of the US Federal Drug Administration for research and community surveil- lance to estimate COVID-19 infectivity within the firefighter workforce. We found that firefighter/paramedics who tested posi- tive were significantly more likely to have greater number of contacts with COVID-19 positive patients as well as spend less time (less than 5 min) with COVID-19 positive patients compared with firefighter/paramedics who tested SARS-CoV-2 antibody negative. Among all firefighters/para- medics who tested SARS-CoV-2 antibody positive, none had reported receipt of the annual influenza vaccine in the 12 months prior to antibody testing. It may be possible that those firefighters/paramedics who tested positive engage in riskier behaviour (ie, inconsistent use of PPE) that could lead to greater risk of exposure. It is possible that an individual’s vaccination behaviour can provide insight into their overall risk tolerance and work-related safety practices. For example, community-based studies evaluating risky sexual behaviour among young adults who were vaccinated against human papillomavirus showed they engaged in less risky behaviours such as being less likely to not use a condom and drink two or more times per week. 16 In our study, we found that PPE use (while not significant) was lower for firefighters who tested positive versus those who tested COVID-19 antibody negative (85.7% vs 94.2%)—more than a twofold difference. A recent systematic review examining clustering and co-occurrence of multiple risk behaviours (ie, drinking, physical activity, diet and so on) found the strongest associations by occupation (up to fourfold increased odds) and by educational attainment. 17 Furthermore, in other occupational groups like the construc- tion workforce, 18 health behaviours have been linked to safety perceptions where obese construction workers with low phys- ical activity were less concerned about job-related injuries.
Caban-Martinez AJ, et al . Occup Environ Med 2020; 77 :857–861. doi:10.1136/oemed-2020-106676
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