RESUMO
@#New case is a basic concept in epidemiology and often serves as the numerator for the indexes of the frequency of disease in a population． However，the exact definition of new case is still un- clear． Based on the natural history of disease combined with the concept of event-state，this article rede- fines case-related concepts and compares them with traditional concepts． The study found that the so- called new cases in traditional epidemiological studies are actually the cases firstly discovered，including newly discovered cases and identifiable newly incident cases ( disease initiation) ． In the real world，new cases are often difficult to measure，and together with the continuous process of disease development and iceberg phenomenon，those may affect the accuracy of disease frequency measurement and causal infer- ence． A correct understanding of the meaning of case-related concepts，the meanings they reflect，and the differences between them help to rationally interpret the results of epidemiological research．
RESUMO
The specific fragment of Pneumococcal surface protein A(PspA)and Pneumococcal Surface Adhesin A(PsaA)gene was amplified by PCR from Streptococcus pneumonia 5 and Streptococcus pneumonia 19.The amplified fragnent of PspA and PsaA gene was ligated into pET-27b(+)vector and transformed into BL 21 E.coli for expression and obtain the expressive production of PspA and PsaA.Induced by IPTG,the expression level was as high as 75 % of the total disolube protein.The result showed that the recombinant plasmid could express a specific 75 kDa and 37 kDa fusion protein in E.coli BL 21,which showed the good immunogenicity and a broadly cross reactivity with the other serotypes.