we used 498 anonymized serum samples from persons in qatar with and without dromedary contact ( technical appendix ) and control serum from europe ( national institute for public health and the environment , bilthoven , the netherlands ; and university of bonn , bonn , germany ) . sampling in qatar was cleared by the ethics and institutional animal care and use committees of the medical research center , hamad medical corporation ( permit 2014 - 01 - 001 ) . samples from the netherlands were used in accordance with the dutch federation of medical scientific associations code of conduct for proper use of human tissue . samples from germany were used in accordance with german national laws . of the 498 samples , cohort a consisted of 109 healthy workers ( 5 camel slaughterers [ subcohort a1 ] and 104 sheep slaughterers [ a2 ] ) at the central slaughterhouse in doha , qatar . all workers lived together and had contact with camels and sheep at the central animal market ( cam ) . cohort c consisted of 22 healthy men living and working at the al shahaniya barn complex near the international dromedary racing track , and cohort d consisted of 155 healthy men living and working on a dromedary farm in dukhan , western qatar ; molecular data showed ongoing circulation of mers - cov in dromedaries in these locations ( technical appendix ) . cohort f consisted of 10 samples from persons working and living at a complex with 200 sheep barns in northern qatar . cohort g consisted of 138 samples for confirming specificity of the testing algorithm ( 66 samples from the netherlands and germany from persons with recent human cov infection [ subcohort g1 ] and 72 samples from the netherlands obtained for routine testing from persons with suspected bordetella pertussis infection [ g2 ] ) . we used microarray technology as described ( 3,5,6 ) to analyze samples for the presence of igg reactive with mers - cov s1 antigen ( table ) . to avoid overinterpretation of data , we set the reactivity cutoff at 30,000 relative fluorescent units for subsequent analyses ( 6 ) . samples from 20 of 294 persons with camel contact were reactive ; no control or noncontact samples were reactive . among camel handlers at the al shahaniya and dukhan locations , 4 of 22 and 8 of 155 , respectively , had antibodies to mers - cov s1 . at the cam , 1 of 8 handlers had antibodies . at the slaughterhouse location , 3 of 104 sheep slaughterers and 4 of 5 * ger , germany ; hcov , human coronavirus ; mers - cov , middle east respiratory syndrome coronavirus ; na , not applicable ; nl , the netherlands ; nt , not tested ; prnt90 , 90% plaque - reduction neutralization test ; s1 , mers - cov s1 antigen . reactivity of human serum samples , from persons with and without dromedary contact , with s1 antigens of various coronaviruses ( covs ) , qatar , 20132014 . a ) middle east respiratory syndrome cov s1 ; b ) human cov oc43 s1 ; c ) severe acute respiratory syndrome cov s1 . black lines indicate median ; dotted black lines at 30,000 rfu depict cutoff for analysis . human cohorts : a1 , camel slaughterers ; a2 , sheep slaughterers who had contact with dromedaries and camel slaughterers ; b , workers at the central animal market ; c , barn workers at the international camel racing track ; d , workers on camel farms ; e , construction workers ; f , sheep farmers ; g1 , persons recently infected with a common human cov ( serum samples from the netherlands and germany ) ; g2 , persons with suspected bordetella pertussis infection ( serum samples from the netherlands ) . samples from subcohort g1 ( n = 66 ) and from all camel - contact cohorts were tested for antibodies to cov oc43 s1 , a common human cov ; all showed high seropositivity ( range 89%100% ) ( figure ) . all 498 samples were tested for reactivity to severe acute respiratory syndrome cov s1 ; none reacted ( figure ) . we used a 90% plaque - reduction neutralization test ( prnt90 ) to confirm the presence of mers - cov  specific antibodies in serum samples from camel handlers . for testing , we used the 20 samples that were reactive to mers - cov s1 and a random selection of nonreactive samples from camel - contact ( n = 35 ) and noncontact ( n = 48 ) cohorts . results were positive for 10 of the 20 mers - cov s1 antibody  positive samples ( reciprocal titers of 20 or 40 ) ( table ) . all but 1 of the 35 samples from persons with camel contact who had negative s1 elisa screening results were negative by prnt90 ; the positive sample had a reciprocal titer of 20 ( table ) . furthermore , 6 samples from s1-positive and 2 from s1-negative persons with camel contact showed a reciprocal titer of 10 , but titers of 10 were not observed in the noncontact cohorts . five of these 8 reactive samples were also positive in a whole - virus mers - cov immunofluorescence assay at dilution 1:100 ; however , we regarded these as negative to avoid overinterpretation of data ( data not shown ) . we detected mers - cov neutralizing antibodies in healthy persons who had daily occupational contact with dromedaries but not in persons without such contact . only limited evidence is available regarding the presence of mers - cov antibodies in the general human population or in specific population cohorts . however , an overall seroprevalence of 0.15% was found in a cross - sectional study in saudi arabia , and among slaughterhouse workers , neutralizing antibodies were detected in 5 of 140 participants ( 7 ) . this finding is similar to our finding among slaughterhouse workers : 7 of 109 were mers - cov antibody  positive . four other studies lacked serologic evidence of mers - cov infection in humans with occupational exposure to dromedaries ( 811 ) . however , only 1 of those studies documented actual mers - cov circulation in dromedaries during human contact , and it was concluded that mers - cov was not highly transmissible from camels to humans , although only 7 persons had regular contact with only 1 herd ( 8) . on several occasions , the percentage of camels shedding mers - cov was high ( 60% ) at the cam and slaughterhouse ( c.b.e.m . thus , locations with a continuous flow of dromedaries with different places of origin and different immune statuses may enable prolonged circulation of mers - cov and sustained exposure of dromedary handlers to the virus ; in qatar , such locations would include the cam , slaughterhouse , and barns near the international racing tracks .   in this study , prnt90-derived antibody titers were relatively low compared with those from earlier studies of mers patients and dromedaries ( 2 ; b.l . the lower titers might reflect the apparent asymptomatic manifestation of mers - cov infection , individual differences in susceptibility , or both ( 2 ) . also , primary infections may result in a short - lived antibody peak followed by a rapid waning of antibody , depending on virus and host properties ( 12 ) , as seen in influenza a(h5n1 ) virus infection : antibody levels are higher in symptomatic than asymptomatic h5n1-infected persons , and antibodies wane more quickly during asymptomatic infection ( 13 ) . mers - cov antibody kinetics and the persistence of antibodies detected by different serologic methods are not known . such parameters are needed to estimate the force of infection on the basis of serologic data ( 14 ) . mers - cov  seropositive participants in this study did not report severe health problems , giving evidence for frequent unrecognized human infections . assuming the health histories are accurate , this finding implies that the current overall mers - cov  associated death rate of 37.1% ( 1 ) is most likely an overestimation of the actual rate and that most infections may be asymptomatic or mild . a major issue to be resolved is whether , and to what extent , asymptomatic cases contribute to the spread of mers - cov ; it is well recognized that variability in disease transmission exists among humans ( 15 ) .
we determined the presence of neutralizing antibodies to middle east respiratory syndrome coronavirus in persons in qatar with and without dromedary contact . 
 antibodies were only detected in those with contact , suggesting dromedary exposure as a risk factor for infection . 
 findings also showed evidence for substantial underestimation of the infection in populations at risk in qatar .