all surviving members of the 2012 outbreak in jordan , their exposed contacts , and their household members who were identified serologically as either mers - cov positive or indeterminate were asked to consent to further participation .
participants were asked to provide a follow - up serologic specimen so we could compare 34-month results with 13-month results .
specimens were prepared by the jordan central public health laboratory ( amman , jordan ) and tested at the us centers for disease control and prevention ( atlanta , ga , usa ) .
antibody titers in serum samples were determined by an anti  mers - cov nucleocapsid indirect elisa and by mers - cov ( hu / jordan - n3/2012 strain ) indirect ifa ( 1 ) . the presence of neutralizing antibody titers was determined by microneutralization with live mers - cov ( hu / jordan - n3/2012 strain ) in a biosafety level 3 laboratory as described previously ( 1 ) .
neutralization titers were defined as the reciprocal of the highest serum dilution completely protecting the vero cell monolayer from cytopathic effect in at least 1 of 3 parallel wells .
of the 15 surviving persons with > 1 positive serologic test result , 13 ( 87% ) consented to follow - up testing .
all 7 ( 100% ) surviving persons with > 2 positive serologic test results 13 months after the mers - cov outbreak also consented .
each of these 7 persons was considered to be a probable mers - cov case - patient according to world health organization criteria ; each had had a symptomatic acute respiratory infection during the outbreak period and documented , unprotected exposure to > 1 person with a case confirmed by reverse transcription pcr .  
for the 7 probable case - patients , elisa titers at 34 months ranged from < 400 to 1,600 , representing reduced antibody titers compared with the 13-month estimates ( 4006,400 ) for all but 1 person ( table ) . a nurse who worked in an intensive care unit ( participant 06 ) and cared for confirmed case - patients during
the outbreak was the only participant for whom elisa indicated a consistent titer of 1,600 at both times .
* ifa , immunofluorescence assay ; ind , indeterminate ; mn , microneutralization assay ; na , not applicable ; nd , not documented ; neg , negative ; pos , positive . 
 symptomatic but refused hospitalization .  classified overall as being serologically negative at 34 mo .   of these 7 participants ,
6 ( 86% ) had neutralizing antibody titers ranging from 20 to 80 at the 34-month follow - up evaluation , and only 2 ( 29% ) had any decrease in neutralizing antibody titers over time .
of the 7 participants for whom ifa results were positive at 13 months , 4 ( 57% ) had positive results at 34 months .   neutralizing antibody titers against middle east respiratory syndrome coronavirus ( hu / jordan - n3/2012 strain ) among 7 surviving case - patients at 13 and 34 months after the 2012 outbreak in jordan .
for the 8 surviving participants whose serologic results at 13 months were indeterminate , 6 ( 75% ) consented to further testing .
of these , 3 ( 50% ) reported having had no respiratory symptoms , 1 reported having had mild respiratory symptoms , and 2 had been hospitalized with respiratory infections ; all 6 had had documented , unprotected exposure to > 1 case - patient .
each of these 6 persons had negative serologic test results at 34 months and continue to be considered negative overall .
antibodies against mers - cov , including neutralizing antibodies , persisted in 6 ( 86% ) of 7 persons 34 months after the 2012 mers - cov outbreak in jordan .
the observed persistence of these antibodies contributes to the understanding of individual immune responses to mers - cov infection , of population - based immunity in regions where mers - cov outbreaks have occurred , and to efforts for developing effective vaccines and therapeutics to counter mers - cov infections .  
notwithstanding improvements in public health awareness and infection control practices in affected countries on the arabian peninsula and in the middle east , emergence of the virus ( e.g. , its introduction to south korea and the resultant epidemic of 2015 ) ongoing . 
mers - cov continues to pose grave risks to international healthcare and socioeconomic systems ( 6 ) .
it has been hypothesized that mild or asymptomatic mers - cov infections are potentially associated with lower levels of mers - cov neutralizing antibodies over time ( 7 ) .
all 7 case - patients reported here had respiratory symptoms , were relatively young , and had few underlying medical conditions ( table ) .
any association between our mers - cov antibody results and clinical severity is therefore difficult to assess . nonetheless , of the 5 persons for whom chest radiographs showed substantial changes within 3 days of symptom onset , each remained positive by microneutralization ( > 20 ) 34 months after the outbreak .
although some similarities in the short - term development of antibodies against mers - cov and sars - cov ( e.g. , seroconversion 23 weeks after illness onset ) have been observed ( 8,9 ) , longer term serum antibody kinetics of these infections have not yet been compared .
after sars - cov infection , robust igg titers were observed through the second year but declined substantially during the third year after infection ( 10 ) .
our finding of generally reduced but persistent mers - cov antibody responses even at 34 months suggests the potential for longer lasting antibody - mediated protective immunity against reinfection . however , whether such long - lasting antibodies can prevent reinfection or affect clinical outcome has yet to be examined .
diverse individual antibody test results allude to a potential role of genetic factors in explaining observed differences in immunologic responses to mers - cov exposure and infection .  
the times at which mers - cov antibodies were measured in our study were chosen because of logistics and field practicalities .
although limited outbreaks of mers - cov have occurred in jordan since 2012 , contact tracing efforts by investigators in jordan lead us to believe that these persons were not subsequently exposed .
the observed elisa titers and neutralizing antibody titers support this supposition ; otherwise , we would expect increases resulting from a booster effect after secondary exposure and infection . to further assess the duration and resiliency of mers - cov antibodies in human populations , continued follow - up serologic evaluations of these persons
 to determine how long antibodies against middle east respiratory syndrome coronavirus persist , we measured long - term antibody responses among persons serologically positive or indeterminate after a 2012 outbreak in jordan . 
 antibodies , including neutralizing antibodies , were detectable in 6 ( 86% ) of 7 persons for at least 34 months after the outbreak . 
The Study
Conclusions