Initiative for Vaccine Research (IVR)

Presentations

Clinical aspects and pathogenesis of SARS

Presentations from Canada and Hong Kong stressed that SARS occurred against a routine background of numerous patients suffering from atypical pneumonia, approximately half of whom never had a specific etiology attributed to their illness, and with many patients suffering from multiple concurrent infections. That situation represented a diagnostic challenge to the clinician and made it difficult to differentiate SARS from other causes of atypical pneumonia. In addition, the increased mortality rate for SARS seen among older age groups of patients is not unlike that seen for patients in this age group suffering from atypical pneumonia due to other causes. These observations support the need for a vaccine that will be effective in older populations, where it is well known that immune response to vaccination may not be as robust as that seen in younger age groups. A preliminary report on the analysis of chemokine and cytokine responses to SARS infection, as measured by gene expression profiles, suggested that elevated levels of certain cytokines may be indicative of the outcome of infection, and a better understanding of these responses may offer suggestions into treatment options as well a potential target for therapeutic intervention.

Coronavirus biology and genetic diversity

The availability of infectious clones of the SARS coronavirus (CoV) was discussed, as well as options that this may offer for future vaccine development efforts. New results were shared on sequence divergence of SARS CoV isolates recovered during the different outbreaks, and it became clear that virtually all cases detected in Canada, Vietnam, Taiwan and Singapore were either genetically similar or identical to those resulting from the super-spreader event documented at "Hotel M" in Hong Kong. Recent results of sequence analyses on isolates from patients from Hong Kong who were not associated with the Hotel M outbreak, as well as emerging data from China, suggest that other genetically diverse strains of SARS CoV exist and are capable of causing a SARS-like illness. Additional information on the genetic and antigenic differences among all isolates is urgently needed to ensure that vaccine candidates are effective in protecting against all SARS CoV strains. These observations can also provide additional molecular tools for epidemiological investigations, as was demonstrated during the consultation.

Animal models for SARS

A growing body of information suggests that non-human primates of various species are susceptible to infection with SARS CoV, but none tested to date appear to faithfully replicate the severe illness seen among human SARS patients. Cynomolgus monkeys experimentally infected with the SARS CoV develop a multifocal pneumonia, with evidence of infection noted by immunohistochemical analysis of macrophages and virus visualized by electron microscopy on samples taken from the throat and nasal passages. A specific immune response was detected in sera drawn from about day 10 post inoculation onward, and some monkeys developed a rash during acute infection. Similar results were seen when rhesus monkeys (Macaca mulata) were experimentally infected; no significant disease was seen, although monkeys did have mildly elevated temperatures and appeared sick. No evidence of viremia was seen, but 5 of 8 animals were positive for SARS CoV in lung and kidney tissues. Their immune response was characterized by low titers of neutralizing antibodies and evidence of a cellular immune response. Pathological examinations revealed typical interstitial pneumonia. No evidence of transmission of SARS CoV was seen from infected to non-infected monkeys housed in the same room. Younger animals (2.5 to 3.0 kg) appeared to be more susceptible to infection as compared to older, larger animals. Comments made during discussions suggested that African green monkeys (Cercopethicus aethiops) might also be susceptible to SARS CoV infection, however, no results were presented during the consultation to support this observation. Recent studies suggest that cats and ferrets are susceptible to SARS CoV infection and that ferrets develop a robust respiratory infection leading to disease. They stop eating soon after infection and suffer significant weight loss culminating in death around day 16 post-infection. Evidence of SARS CoV was found in several tissues and specimens taken from infected animals. While additional validation of this animal model is still required, it appears that the ferret may be valuable as a model for SARS CoV infection. The need for a standard challenge dose and consistent measurement of indicators of infection were discussed and agreed upon.

Experience with animal coronavirus vaccines

Several veterinary vaccines have been developed for use against coronavirus infections in domestic animals, and some of these were discussed. Of greatest concern is the experience with inactivated vaccines against for feline infectious peritonitis virus (FIPV) infection, where administration of an inactivated vaccine sensitized cats to subsequent antibody dependent enhancement of infection, leading to more severe peritonitis and death. The spike protein of FIPV appeared to be responsible for enhanced infection of macrophages that resulted in rapid onset of acute peritonitis. The importance of determining if similar events occur with candidate SARS CoV vaccines in humans was underlined, and experiments to test this through passive antibody transfer and subsequent challenge in non-human primates were discussed. Information was also presented regarding vaccines to protect chickens against avian infectious bronchitis virus (IBV). Examples were cited where host genetic variation may lead to variable response to vaccination using an attenuated strain. Multiple serotypes of IBV are known to exist, and relatively poor heterologous protection between serotypes has been reported. These observations support the recommendation to obtain a comprehensive database to facilitate a better understanding of genetic and antigenic diversity among SARS CoV prior to final selection of candidate vaccines.

Strategies for SARS vaccine development

Several options exist for SARS CoV vaccine development and these were discussed during the consultation. Traditional inactivated vaccines are likely to be the easiest to produce both in terms of cost and time to produce, but the need to determine if antibody mediated enhancement occurs was stressed. Live attenuated vaccines may generate the best overall protection, but concerns about possible recombination or reversion to wild-type must be addressed. Subunit vaccines employing major gene products (such as spike protein) also hold promise as effective immunogens, with or without the addition of adjuvants. Experience gained in attempts to develop vaccines against HIV/AIDS or malaria have resulted in significant advances in the use of prime/boost strategies for immunization using DNA and vectored vaccines (vaccinia, adenovirus vectors for example), and these strategies hold promise for SARS vaccine development as well. The consensus opinion was that, to the largest extent possible, a variety of different candidate vaccines should be developed, at least through preclinical evaluation and comparison in animal models.

Progress in the development of candidate SARS vaccines

Both major vaccine manufacturers and smaller biotech firms have made significant progress in the initial development of candidate vaccines for SARS CoV. That was facilitated by the provision of funding by the U.S. National Institutes of Health to several companies to begin developmental efforts. Most manufacturers are using a SARS CoV seed virus isolate provided by the Centers for Disease Control and Prevention (CDC). This isolate (UTAH strain) was made from sputum from an acutely ill US traveler exposed in Hong Kong. The isolation was made under formal Good Laboratory Practices (GLP) using certified cells and media provided to CDC for this purpose by Aventis Pasteur. The isolate was fully sequenced and shown to be virtually identical to the Urbani strain of SARS CoV, and seed virus is available to manufacturers possessing appropriate containment facilities on request to CDC. Most manufacturers are growing the virus in certified Vero cells with serum-free media and report good growth to high titers. Many will start with the inactivated vaccine candidates, although alternate strategies including DNA, vectored vaccines, subunit products and live, attenuated candidates are also under consideration or development. Vaccine manufacturers were encouraged to continue their developmental efforts using differing vaccine strategies, to ensure that the maximum number of options will be ultimately available. At least three different vaccine candidates (Sino-3, BJ-01 and GZ) are under development in China, and Phase 1 clinical trials may begin soon; other manufacturers anticipate clinical trials with inactivated vaccines to begin within 12-18 months.

Regulatory considerations

Regulatory officials from Canada, China and the USA attended the consultation and offered their suggestions as to how SARS vaccine development might progress most rapidly yet consistently with regulatory requirements for licensure. In general, all recommended that already approved cell lines and reagents, as well as validated and approved procedures and licensed facilities be used whenever possible to eliminate the need for costly and time-consuming validation of essential ingredients and processes. Manufacturers were encouraged to open and maintain an active dialogue with regulatory officials, starting early in the development process.

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