during the epidemic , close contacts were prospectively identified by the hong kong special administrative region government department of health through standardized telephone interviews with all 1,755 confirmed sars patients within 1 week of hospital admission ( february 15june 22 , 2003 ) .
a close contact was defined as a person who had cared for , lived with ( in the same household ) , or came into direct contact with body fluids of the sars patients within 10 days before hospital admission .
a total of 3,612 close contacts were recorded ; 505 were diagnosed as having sars . of the remaining 3,107 contacts , 2,805 ( 90% ) had a telephone number available , as reported by the primary patient .
we successfully contacted 2,337 ( 83% ) from october 23 to november 30 , 2003 , and 1,776 ( 57% of those eligible ) consented to a telephone interview after the purpose of the study was explained to them by trained public health nurses .
the interview consisted of questions that assessed the relationship between the patients and contacts ; the timing , intensity and frequency of contact ; precautionary measures adopted during contact with the patient ; known contact with other sars patients ; clinical symptoms of febrile , respiratory , gastrointestinal , or constitutional illness since february 2003 ; medical and travel history ; and sociodemographic details .
$ 25.00 ) were given to participants after blood was collected as compensation for time and travel costs .
samples were screened by the government virus unit of the department of health by using viral lysate enzyme - linked immunosorbent assay ( elisa ) ( gbi biotech , beijing ) .
positive results were confirmed with immunofluorescence assay ( ifa ) and neutralization tests . for the ifa , microscopic slides coated with sars - cov 
a positive test is indicated by cytoplasmic fluorescence under uv microscopy . by using ifa as the standard
, the elisa detects antibody with ifa titer of > 25 ( i.e. , sensitivity of 100% ) and has a specificity of 95% .
neutralization test was performed by standard virologic method with vero e6 cells and sars - cov isolate 6109 .
the reported sensitivity of 100% was for convalescent - phase serum samples taken a few weeks after the onset of infection in sars patients , which should apply to our study . during the early phase of infection
therefore , the sensitivity is 80%90% ( depending on the number of days after illness onset when the serum samples were taken ) .
however , this sensitivity should not have affected our findings , which were based on tests carried out at least 6 months after the last reported case of sars in hong kong .
the study received ethics approval from the department of health ethics committee , which complies with the declaration of helsinki .
of the 1,068 samples analyzed , 2 ( 0.19% , exact 95% ci 0.02%0.67% ) had a positive titer ( 1:25 to 1:50 on ifa compared to at least 1:100 in most recovered sars cases ) for sars - cov igg antibody .
neither participant with a positive sample reported a chronic medical condition or being sick with febrile or respiratory illness from february to august .
both seropositive participants arose from two superspreading events in hong kong , i.e. , prince of wales hospital nosocomial outbreak and amoy gardens environmental point source community outbreak ( 1,3 ) .
the contact of the prince of wales hospital seropositive participant reported one other close contact , who was interviewed but declined to be tested .
the other seropositive index patient living in amoy gardens was separately identified by three intrafamilial index patients , all of whom lived in the same household and reported only each other as close contacts .
the participants who consented to testing were broadly similar to those who declined , except that the first group had relatively fewer children and fewer of the first group were men ( table ) .
however , those who consented to testing were more likely to report more frequent contact and closer relationships with sars patients , more febrile or respiratory illness episodes since february , and a travel history to sars - affected regions , which may have biased our seroprevalence estimate upwards .
includes washing hands before touching mouth , eyes , and nose ; washing hands with soap ; wearing face mask ; using serving utensils during meals ; adopting precautionary measures when touching possibly contaminated objects , washing hands after touching possibly contaminated objects ; adopting home preventive measures ( such as maintaining good ventilation and using bleach to clean surfaces and home appliances ) against sars ; and adopting workplace preventive measures ( such as maintaining good ventilation , using bleach to clean surfaces and office furniture , and not allowing staff who are sick to come to work ) against sars .
the extent of seroconversion in close contacts of confirmed patients should provide the upper limit of sars - cov antibody seroprevalence in the general population , given the relatively intense exposure history of these persons to sars patients .
our finding of the near absence of transmission resulting in asymptomatic infection in this representative high - risk group of close contacts indicates that the prevailing sars - cov strains in hong kong almost always led to clinically apparent disease .
whereas some sars patients , especially healthcare workers , might have been initially admitted to reduce transmission to family members , virtually all sars patients ( perhaps with very few exceptions in children ) had severe disease requiring inpatient treatment ; thus , we can infer that infection with sars - cov inevitably caused severe disease requiring hospitalization . although our results suggest that sars - cov was a new virus in humans without a close precursor or an antigenically related virus that would have induced at least a small degree of cross - reactivity on serologic testing , a recent study on a select group of 938 healthy hong kong adults , whose serum had been previously stored as part of a hepatitis b serosurvey in 2001 , indicated that 1.8% of the sample had acquired a sars - cov  related virus infection at least 2 years before the 2003 sars outbreak ( 5 ) .
the investigators speculated that the virus that affected these healthy , seropositive persons was antigenically closer to the recently isolated animal sars - cov  like virus ( 3 ) than human sars - cov , but interspecies transmission from animals to humans was probably inefficient , as the virus might not have adapted in the new host .
this hypothesis would explain why only a few persons became infected and why they were likely to have been asymptomatic .
this hypothesis would be compatible with the presumed asymptomatic infection observed in guangdong animal traders , especially in those who handled masked palm civets , who had a seropositivity rate of 72.7% ( exact 95% ci 49.8%89.3% ) in the absence of prior overt clinical disease ( 6 ) .
the limitations of the study include incomplete contact tracing , especially in the earlier parts of the epidemic , and potential recall bias from underreporting of contacts by some patients who were too sick to answer questions .
another possible shortcoming is the lack of a survey of close contacts who did not report a telephone number , although there is no reason to suspect they had a systematically different serologic profile .
in fact , these were mostly nonhousehold contacts who would have had less intense exposure to sars patients .
in addition , because peak infectivity , as indicated by viral load , usually occurred during week 2 of illness ( 7 ) , when most of the patients would have been isolated in hospital ( the mean onset - to - admission interval decreased from a maximum of 9.3 days in late february to 1.0 day by mid - may ) ( 8) , transmission to close contacts in the later stages of the epidemic was less likely .
finally , contacts who refused to participate ( 561 ) or refused to have serologic testing ( 708 ) might have done so because they were concerned about having had sars ( possibly because of having had sars - like symptoms ) and did not want to be identified and stigmatized as having been infected with sars - cov .
surveys in other countries with large - scale outbreaks such as canada , china , singapore , and taiwan should be undertaken to confirm our findings .
of the 1,068 samples analyzed , 2 ( 0.19% , exact 95% ci 0.02%0.67% ) had a positive titer ( 1:25 to 1:50 on ifa compared to at least 1:100 in most recovered sars cases ) for sars - cov igg antibody .
neither participant with a positive sample reported a chronic medical condition or being sick with febrile or respiratory illness from february to august .
both seropositive participants arose from two superspreading events in hong kong , i.e. , prince of wales hospital nosocomial outbreak and amoy gardens environmental point source community outbreak ( 1,3 ) .
the contact of the prince of wales hospital seropositive participant reported one other close contact , who was interviewed but declined to be tested .
the other seropositive index patient living in amoy gardens was separately identified by three intrafamilial index patients , all of whom lived in the same household and reported only each other as close contacts .
the participants who consented to testing were broadly similar to those who declined , except that the first group had relatively fewer children and fewer of the first group were men ( table ) .
however , those who consented to testing were more likely to report more frequent contact and closer relationships with sars patients , more febrile or respiratory illness episodes since february , and a travel history to sars - affected regions , which may have biased our seroprevalence estimate upwards .
includes washing hands before touching mouth , eyes , and nose ; washing hands with soap ; wearing face mask ; using serving utensils during meals ; adopting precautionary measures when touching possibly contaminated objects , washing hands after touching possibly contaminated objects ; adopting home preventive measures ( such as maintaining good ventilation and using bleach to clean surfaces and home appliances ) against sars ; and adopting workplace preventive measures ( such as maintaining good ventilation , using bleach to clean surfaces and office furniture , and not allowing staff who are sick to come to work ) against sars .
the extent of seroconversion in close contacts of confirmed patients should provide the upper limit of sars - cov antibody seroprevalence in the general population , given the relatively intense exposure history of these persons to sars patients .
our finding of the near absence of transmission resulting in asymptomatic infection in this representative high - risk group of close contacts indicates that the prevailing sars - cov strains in hong kong almost always led to clinically apparent disease .
whereas some sars patients , especially healthcare workers , might have been initially admitted to reduce transmission to family members , virtually all sars patients ( perhaps with very few exceptions in children ) had severe disease requiring inpatient treatment ; thus , we can infer that infection with sars - cov inevitably caused severe disease requiring hospitalization .
although our results suggest that sars - cov was a new virus in humans without a close precursor or an antigenically related virus that would have induced at least a small degree of cross - reactivity on serologic testing , a recent study on a select group of 938 healthy hong kong adults , whose serum had been previously stored as part of a hepatitis b serosurvey in 2001 , indicated that 1.8% of the sample had acquired a sars - cov  related virus infection at least 2 years before the 2003 sars outbreak ( 5 ) .
the investigators speculated that the virus that affected these healthy , seropositive persons was antigenically closer to the recently isolated animal sars - cov  like virus ( 3 ) than human sars - cov , but interspecies transmission from animals to humans was probably inefficient , as the virus might not have adapted in the new host .
this hypothesis would explain why only a few persons became infected and why they were likely to have been asymptomatic
. this hypothesis would be compatible with the presumed asymptomatic infection observed in guangdong animal traders , especially in those who handled masked palm civets , who had a seropositivity rate of 72.7% ( exact 95% ci 49.8%89.3% ) in the absence of prior overt clinical disease ( 6 ) .
the limitations of the study include incomplete contact tracing , especially in the earlier parts of the epidemic , and potential recall bias from underreporting of contacts by some patients who were too sick to answer questions .
another possible shortcoming is the lack of a survey of close contacts who did not report a telephone number , although there is no reason to suspect they had a systematically different serologic profile .
in fact , these were mostly nonhousehold contacts who would have had less intense exposure to sars patients .
in addition , because peak infectivity , as indicated by viral load , usually occurred during week 2 of illness ( 7 ) , when most of the patients would have been isolated in hospital ( the mean onset - to - admission interval decreased from a maximum of 9.3 days in late february to 1.0 day by mid - may ) ( 8) , transmission to close contacts in the later stages of the epidemic was less likely .
finally , contacts who refused to participate ( 561 ) or refused to have serologic testing ( 708 ) might have done so because they were concerned about having had sars ( possibly because of having had sars - like symptoms ) and did not want to be identified and stigmatized as having been infected with sars - cov . surveys in other countries with large - scale outbreaks such as canada , china , singapore , and taiwan should be undertaken to confirm our findings .
 a total of 1,068 asymptomatic close contacts of patients with severe acute respiratory ( sars ) from the 2003 epidemic in hong kong were serologically tested , and 2 ( 0.19% ) were positive for sars coronavirus immunoglobulin g antibody . 
 sars rarely manifests as a subclinical infection , and at present , wild animal species are the only important natural reservoirs of the virus . 