the sars outbreak in vietnam began with the admission of a traveler from hong kong on february 26 , 2003 , to hospital a , a 56-bed , three - story , privately owned and expatriate - operated facility located in hanoi . within 2 weeks , extensive nosocomial transmission of sars occurred in workers , patients , and visitors in hospital a. on march 12 , hospital a was closed to new admissions except for sick hospital a workers .
on that date , the 120-bed , six - story public hospital b began admitting patients with suspected and probable sars .
hospital b treated 33 patients with laboratory - confirmed sars between march 12 and may 2 , 2003 , the discharge date of the last patient ( figure ) .
of these , 23 were admitted directly to hospital b , and 10 were transferred from hospital a to hospital b on march 28 .
many of hospital b s 33 patients were exposed to sars as patients or visitors in hospital a. laboratory - confirmed cases of severe acute respiratory syndrome ( sars ) by date of admission , in hospital a and hospital b , vietnam , february 
the ten case - patients who were transferred from hospital a to hospital b on march 29 , 2003 , are noted by cross - hatching .
no nosocomial sars - associated coronavirus ( sars - cov ) transmission was reported in hospital b , and none of its 117 healthcare workers ( defined as all staff working in the hospital building during the sars outbreak ) became ill with a sars - compatible illness .
when hospital b began admitting patients , visitors were not tightly restricted , the main elevator was out of service , and families and workers often used the designated patient elevator .
researchers ( k.c.l . , h.q.n . ) and infection control advisors working daily on the hospital b wards reported variable infection control and patient isolation , particularly during the early weeks . on march 19 , formal infection control training was organized and substantial technical support and supplies arrived from who , mdecins sans frontires  belgium , and the japan international cooperation agency .
systems were established to restrict visitors , and entry guards and mdecins sans frontires advisors were tasked with distributing and monitoring personal protective equipment , such as n95 masks , gloves , gowns , and hand sanitizer .
two of the authors of this article ( k.c.l . , h.q.n . ) , who worked daily on the wards , observed that infection control practices improved considerably after these interventions . to help researchers determine whether sars - cov transmission occurred among hospital b healthcare workers , staff were offered serologic testing from may 12 to 14 and were asked to complete a short questionnaire in vietnamese .
participants provided written consent and answered questions about demographics , level of contact with sars case - patients , and personal protective equipment use during the busiest week of patient admissions ( march 1219 ) and the remaining weeks of the outbreak .
serum specimens were analyzed at the national institute for hygiene and epidemiology , hanoi , and at the centers for disease control and prevention , atlanta , by indirect enzyme - linked immunosorbent assay ( elisa ) and indirect immunofluorescence ( ifa ) on vero e-6 cells infected with sars - cov ( 1 ) .
data were double - entered into excel and analyzed with sas version 8.0 ( sas , inc . , cary , nc ) . of 117 hospital b healthcare workers ,
108 participated ( 92.3% response rate ) . according to the hospital director , all 9 nonparticipants remained well , and none had a history of sars - like illness . among participants , 62 ( 57.4% ) respondents worked on the sars wards ( table ) .
most ( 85.5% ) were physicians and nurses . during the first week of sars patient care in hospital b , 39 ( 62.9% ) of sars
ward workers reported working in sars - patient rooms for > 6 hours on their single busiest day .
of the 62 workers , 58.1% and 64.5% reported being in sars patient rooms during medication nebulizer treatment , and 65% reported being in patient rooms during noninvasive positive pressure ventilation .
all but one respondent wore a mask  always  or  usually  while in sars patients rooms .
however , during the first week of sars patient care in hospital b , 43 ward workers ( 69.4% ) reported wearing only a cloth or surgical mask , often in combination .
all 62 sars ward workers reported using an n-95 mask after march 19 , although only 56 ( 90.3% ) reported  always  or  usually  using a mask while in sars patients rooms .
respondents reported using gloves 77.4% of the time before march 19 and 75.8% after march 19 .
reported symptoms and personal health behaviors of healthcare workers are also presented in the table .
one sars ward respondent reported a fever , and less than 23% reported either a cough or sore throat .
first , our survey is subject to recall and reporting bias , because not only was it difficult for respondents to recall behaviors during specific periods within the previous 2 months , but respondents may have been concerned that results could be used to evaluate their performance .
estimates of sars exposures and the frequency of personal protective equipment use among sars ward workers are therefore probably inflated .
second , we collected serum specimens approximately 10 to 12 days after the last sars patients were discharged ; although these patients were discharged after their 5th to 6th week of illness , the minimal chance that a patient shed virus beyond the usual 2- to 3-week period ( 2 ) would theoretically mean that a few participants may have been tested before seroconversion .
a third limitation is our lack of data on hand - washing or sanitizing practices , important means of preventing respiratory virus droplet spread .
the finding of no infection with sars - cov among hospital b workers in the presence of 33 confirmed sars case - patients may support the hypothesis that , in the absence of a superspreading patient or event , most sars patients will not transmit the virus ( 36 ) .
for example , in singapore , 81% of the first 205 reported probable case - patients had no evidence of transmission of clinically identifiable sars to other persons ( 3 ) .
over 35 healthcare workers in our study reported being exposed to a sars patient during events that can potentially generate aerosols ( i.e. , nebulizer treatment or noninvasive positive pressure ventilation ) , yet they did not acquire sars .
although likely many factors contributed , we demonstrated a lack of sars transmission both before and after the provision of formal infection control training and personal protective equipment .
contrasting the hospital b situation with that of neighboring hospital a may be helpful ; in hospital a , extensive transmission clusters followed admission of the index case - patient .
the 23 directly admitted hospital b patients were less severely ill than the 38 hospital a patients . in vietnam ,
the best available measure of relative disease severity is the death rate and the maximal level of respiratory assistance provided .
although no hospital b patients died or received invasive mechanical ventilation , four received biphasic intermittent positive airway pressure .
seven hospital a case - patients were intubated ; an additional two received biphasic intermittent positive airway pressure .
five hospital a case - patients died in vietnam , and the index case - patient died in hong kong ( 7 ) .
hospital a workers did not wear masks in the earliest days after the index case - patient was admitted , although shortly after the recognition of this nosocomial cluster , enhanced infection control measures were initiated .
in contrast , by the time patients were going to hospital b for evaluation , both patients and healthcare workers were wearing masks ( n.t .
van , pers . comm . ) hospital a nursing staff likely also had longer and closer contact with sars patients . in nursing style ,
hospital b resembled those of other public hospitals in vietnam , where nursing is traditionally a shared function with family members .
families of sars patients in hospital b were observed by authors ( k.c.l . , n.q.h . ) to be feeding , bathing , and toileting the patients .
hospital a nurses , however , were required by hospital guidelines to assume most patient care functions traditionally shared with the patient s family ( l.t .
comm . ) , thereby increasing their direct contact with sars patients and their respiratory and other secretions .
furthermore , the more severely ill sars patients of hospital a likely required more intensive nursing care , perhaps increasing the duration and dose of sars - cov exposure . environmental conditions at the two hospitals differed , but the impact of these differences on sars transmission is unclear .
hospital a was a more modern facility ; however , hospital b had designated sars isolation wards and large spacious rooms with high ceilings and ceiling fans and large windows kept open for cross - ventilation .
in contrast , hospital a s rooms were smaller , and individual air - conditioning units were in use early during the outbreak .
in addition , hospital a had diverse patients ( maternity , postoperative , pediatric , etc . ) housed on the same hospital floor when the sars outbreak began .
the findings of lack of transmission among hospital b healthcare workers raises the question of whether family caregivers or visitors might have become infected with sars - cov , and about the relative infectiousness of hospital b patients in general .
although overt sars transmission to visitors occurred in hospital a , no such transmission to visitors was observed in hospital b. we lack adequate data to quantify the exposure of visitors to patients at either hospital , but the authors who were present ( k.c.l . , h.q.n . )
noted that after the first week , most hospital b family members tended to always wear masks and to rarely use gloves .
studies assessing the serologic status of family and community contacts of case - patients are ongoing .
although community transmission did not seem to play a major role in the vietnam sars outbreak , at least two episodes are known in which sars transmission occurred outside the hospital setting .
this visitor was severely ill and was later hospitalized at hospital b on day 10 after symptom onset ; he is known to have transmitted infection to one contact in the 4 hours immediately before his admission .
if sars viral shedding peaks on day 10 of illness and continues for 23 weeks ( 3 ) , we can assume that some of the hospital b patients were still infectious during their hospitalization . among the 23 directly admitted hospital b patients , the median days to admission was 7 ( range 113 ) after illness onset . in conclusion
, we found no evidence of sars - cov transmission among hospital b workers , despite contact with laboratory - confirmed sars case - patients and variable infection control practices and use of personal protective equipment .
this finding may be explained by differences in infection control practices , use of personal protective equipment ( including masks for patients as well as healthcare workers ) , nursing style , environmental features , and clinical factors such as severity of illness and the absence of a highly infectious sars - cov spreader .
more study is needed to determine how each of these factors affects the risk of sars transmission if we are to adequately prepare for future sars epidemics .
 the severe acute respiratory syndrome ( sars ) outbreak in vietnam was amplified by nosocomial spread within hospital a , but no transmission was reported in hospital b , the second of two designated sars hospitals . 
 our study documents lack of sars - associated coronavirus transmission to hospital b workers , despite variable infection control measures and the use of personal protective equipment . 