Abstract
Objectives-To ascertain all cases of paralytic poliomyelitis in England and Wales during and to determine the source of infection in each case.
Design Results-Of 54 suspected cases of poliomyelitis, 33 were excluded, leaving 21 cases, of which 13 were vaccine associated (nine recipient and four contact) cases, five were imported cases, and three were cases whose source of infection was unknown. No cases due to indigenous wild polioviruses were identified; two were imported cases due to wild viruses. One patient died during the acute phase of the illness, and two children with previously unrecognised severe congenital immune deficiency died between one and two months after the onset of paralysis after the first or second dose of oral polio vaccine. The estimated risk of vaccine associated paralysis is 1-46 per million for the first dose, 049 for the second, zero for the third and fourth doses, and 033 for the fifth.
Conclusions-Indigenous wild poliovirus seems to have been eradicated, although wild virus may be imported; improved surveillance of suspected cases including immediate notification and characterisation of the virus to ensure that eradication is maintained is essential.
Introduction
In Cerebrospinal fluid was collected and assayed for antibodies to poliovirus in five cases, and in three a low titre of antibody was detected. In one (case 10) there was a raised viral index, suggesting intrathecal production of poliovirus antibody.
The NHS operates large scale screening services for carcinoma of the breast and cervix and population screening for osteoporosis is being evaluated in several research centres. To be successful screening requires accurate identification and efficient persuasion of the target population.
Screening by invitation is usually more effective than either opportunistic screening' or self referral,2 but which method of invitation best combines good response rates with low cost is uncertain. Only two basic methods of invitation have been reported: fixed appointments with provision to alter the appointment if required and open ended invitations. Government guidelines for breast cancer screening suggest that specific appointments are preferred.3 Many studies have assessed the performance of both methods individually,4'" and in the few studies that have compared the two specific invitations consistently outperformed open ended invitations. '2-'4 Response rates to fixed invitations rarely exceed 80%, and so material and human resources are wasted. We compared three different invitation methods for compliance and efficiency within a larger study of population based screening for future osteoporosis by dual energy x ray absorptiometry. At present population screening for osteoporosis remains controversial, and this paper does not address this issue.
Discussion
The eradication of poliomyelitis requires high levels of vaccine coverage and adequate surveillance to detect every case. Although poliovaccine coverage in England and Wales has now reached 90% (Department of Health, unpublished data), in some districts coverage is below 70% and many individuals remain unprotected. It is recommended that no adult should remain unimmunised against poliomyelitis.9 No cases of indigenous wild poliomyelitis were identified in this review, although the source of infection could not be found in three cases. Importation of wild virus still occurs, with the potential for transmission among unimmunised subgroups of the population such as itinerants.'°Poliovirus isolates from healthy children and from environmental samples should be characterised to guarantee the absence of circulating wild poliovirus. Measures are also required to improve the surveillance of suspected cases. Only nine of the 19 cases in this review were officially notified. Immediate notification of all suspected cases is essential, both to enable timely control measures to be initiated and to maintain adequate surveillance. Since 1991 surveillance of poliomyelitis has been improved by adding acute flaccid paralysis in children to the list of conditions reportable to the British Paediatric Surveillance Unit."
The diagnosis of poliomyelitis should be considered in any patient with a bulbar or flaccid paralysis and appropriate laboratory investigations undertaken. This study confirms the importance of early collection of faecal specimens. As shedding of the virus may be intermittent at least two samples collected 24-48 hours apart should be obtained as soon as possible after onset of symptoms. Throat swabs should also be taken if the patient is seen within 10 days of the onset of illness, although they did not prove useful in this series. It is important that all isolates from suspected cases should be fully characterised. Serological methods based on monoclonal antibodies and cross absorbed sera are available in the United Kingdom at the PHLS Virus Reference Laboratory and the National Institute for Biological Standards and Control, and the results with both methods show good agreement.
WHO now places less emphasis on detecting neutralising antibodies to poliovirus in suspected cases.5 As this study shows, however, this can provide valuable information so we do encourage the collection of acute and convalescent sera. Specimens of cerebrospinal fluid should be obtained from all cases of meningoencephalitis. Although poliovirus is rarely isolated from cerebrospinal fluid, the specimen is necessary for full assessment of the cases. We were unable to establish the value of detecting intrathecally synthesised poliovirus antibodies.'2 However, a full evaluation of this diagnostic approach is merited, and the potential value of the polymerase chain reaction to diagnosis of poliomyelitis has yet to be assessed. These developments mean that cerebrospinal fluid may become more diagnostically useful in future. Finally, necroscopic specimens (tissues from the brain stem, spinal cord, and descending colon) should be obtained from fatal cases, if possible.
Five imported cases were identified in this series.
Although the risk to travellers is small, ' 
Subjects and methods
The study was approved by the local ethics committee and took place in 1991 at the osteoporosis screening unit, City Hospital, Aberdeen. A single Norland XR26 bone densitometer operated by one full time radiographer was used to scan up to 10 patients a day at the hip and lumbar spine; each scan took about 10 minutes and women were booked in at half hour intervals.

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