Prevalence of antibodies against the Middle East Respiratory Syndrome coronavirus, influenza A and B viruses among blood donors, Saudi Arabia

Sir,

Convalescent plasma therapy has been proposed as a therapy for the Middle East Respiratory Syndrome coronavirus (MERS-CoV) and for influenza A and B viruses.[1,2,3] As part of a protocol for collecting convalescent plasma, we have reported low seroprevalence of MERS-CoV antibodies among cohorts of patients with suspected or laboratory-confirmed MERS-CoV infection, health-care workers, and household contacts exposed to MERS-CoV.[4] Subsequently, we further screened healthy Saudi male blood donors as a potential source for convalescent plasma. We tested 170 consecutive donors aged 18–32 years in the 1^st week of October 2016 for anti-MERS antibodies and 100 consecutive donors for influenza A and B antibodies. The influenza virus A IgG enzyme-linked immunosorbent assay (ELISA) and influenza virus B IgG ELISA were used for qualitative antibody testing according to manufacturer's instructions (NovaTec Immundiagnostica, Dietzenbach, Germany). Briefly, serum samples were diluted 1 in 100 and incubated with ELISA wells coated with influenza A virus antigen (prepared from the strain A/Texas/1/77 [H3N2]) and the influenza B virus antigen (prepared from the strain B/Hong Kong/5/72). Binding of serum IgG to influenza antigens was detected by the addition of horseradish peroxidase-labeled antihuman IgG conjugate followed by the incubation with the substrate (tetramethylbenzidine). Intensity of the developed color (at 450 nm) was measured, which was proportional to specific IgG titers. The ELISAs also offered semi-quantitative evaluation of the results using the arbitrary units “NovaTec-Units” (NTU). The NTUs were calculated with the following formula: Subject's mean absorbance ×10/cutoff = NTU. NTU >11 was considered positive and <9 was considered negative otherwise borderline. IgG MERS-CoV antibodies were tested by ELISA according to manufacturer's instructions (Euroimmun AG, Lübeck, Germany). Results were reported as optic density (OD) ratio, which is calculated as the OD value of the patient's sample divided by the calibrator OD value. Cutoff values recommended by the manufacturer are a ratio of<0.8 is negative, >0.8 and<1.1 is borderline, and a ratio of >1.1 is positive. Most of our cohort (64%) tested positive for IgG Influenza A whereas 35% of them were positive for influenza B antibodies [Table 1]. The titers of anti-influenza A were higher than that of anti-influenza B (15.3 vs. 10.6 NTU, t-test; P < 0.001). None of 170 donors tested for MERS-CoV antibodies were positive.

Table 1.

Prevalence of IgG Middle East Respiratory Syndrome coronavirus, influenza A and B in a Saudi cohort of blood donors from Riyadh, Saudi Arabia

The absence of MERS-CoV antibodies in humans is consistent with our previous findings and others, which showed very low prevalence of MERS-CoV antibodies in Saudi Arabia, even among camel workers.[4,5] The human data are surprising and are in sharp contrast with the data on camels which showed >80% seropositivity among camels from Saudi Arabia, other Gulf states, and Africa.[3] The reasons for this apparent disparity are unclear, but it might suggest that humans, unlike camels, rarely get infected with MERS-CoV. Alternatively, it may be related to different immunologic responses between camels and humans, for example, infection may lead to long-lasting antibody levels in camels but not in humans. We have shown that a critically ill patient with MERS-CoV infection mounted high antibody response during the acute illness; however, the antibody levels declined rapidly during his recovery and disappeared in 8 months.[1] These observations appear to contrast with the high influenza antibody prevalence, which may reflect the high prevalence of infection with influenza in Saudi Arabia along with long-lasting antibody levels to infection or vaccine. Our data on influenza in blood donors are consistent with what has been observed in other countries. For example, a study from Germany found IgG for influenza A among 96% and for influenza B among 98% of blood donors.[6] Our findings demonstrate that plasma obtained from healthy blood donors in Saudi Arabia does not have sufficient antibody titers that allow the use of convalescent plasma for treating patients with MERS-CoV infection. However, convalescent plasma may be a viable option for treating severe influenza in Saudi Arabia, as this option is being currently evaluated in a clinical trial in the United States.[7]

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Conflicts of interest

There are no conflicts of interest.