• Vaccine Name: Plague vaccine USP
• Target Pathogen: Yersinia pestis
• Target Disease: Plague
• Tradename: Plague vaccine USP
• Manufacturer: Cutter Biological Inc
• Vaccine Ontology ID: VO_0000847
• Type: Inactivated or "killed" vaccine
• Status: Licensed
• Preparation: USP consists of an aqueous suspension of virulent P.pestis grown on agar medium in Roux bottles. The organisms are harvested in physiological saline and killed by adding formol to an overall concentration of 0.5 %. The standardized suspension is supplied in 20 ml vaccine vials, each ml containing 2*10^9 bacilli (Meyer, 1970).
• Virulence: Side effects, such as malaise, headaches elevated temperature and lymphadenopathy occur in approximately 10% of those immunised with killed whole cells vaccines (Titball et al., 2001).
• Description: The earliest report of a killed whole cell vaccine against plague was in 1897, developed by HalIkine in India. The vaccine caused severe side-effects and was discontinued, although the HalTkine Institute now produces a formaldehyde-killed preparation of strain 195/P (Russell et al., 1995). It was not until 1946 that a killed whole cells vaccine was developed for use in man. Various methods of killing the bacterial cells have been used, including formaldehyde and heat treatment. The vaccine is currently produced by the Commonwealth Serum Laboratories in Australia and is usually given as a course of three doses over a period of two months(Titball et al., 2001). The vaccine currently used in the USA and in the UK is the Plague vaccine, USP (Cutter Biological), a formaldehyde-killed preparation of the highly virulent 195/P strain of Y pestis (Russell et al., 1995).There are no definitive clinical trials which demonstrate the efficacy of killed whole cell vaccines. However, studies in several animal species have demonstrated protection against bubonic plague. Also there is circumstantial evidence for the efficacy of the vaccine in humans derived from data on the incidence of bubonic plague in immunised US servicemen serving in Vietnam during the period 1961–1971. It is possible that differences in the lifestyles of servicemen and Vietnamese civilians were responsible for the reduced incidence of plague in the former group. Evidence for the efficacy of killed whole cells vaccines for the prevention of pneumonic plague is less conclusive. Cases of pneumonic plague have been reported in individuals immunised with this vaccine. More recently it has been shown that mice immunised with this vaccine are protected against subcutaneous challenge, but not against inhalation challenge with Y. pestis. Together, these findings suggest that killed whole cell vaccines do not induce a response which provides protection against pneumonic plague (Titball et al., 2001). Additional problems of the vaccine include the following: production requires special containment; the vaccine is expensive; protection is short-term and annual boosters are required; the incidence of side-effects is high, local reactions occur in 11-24% of vaccinees and systemic effects occur in 4-10% of vaccinees (Russell et al., 1995).

Mouse Response

• Host Strain: Balb/C (Charles River), NIH/S (Harlan Olac Inc.), outbred Porton strain (bred in-house).
• Vaccination Protocol: Mice, immunized with the Plague vaccine, USP, were given 0.1 ml of neat vaccine (1.8-2.2 X 10’ formaldehyde-killed bacilli) on day 0 and a booster of identical volume on day 8 (Russell et al., 1995).
• Persistence: In all cases, animals showed signs of infection within 24-48 h of challenge with the high doses of Y pestis. The first deaths occurred within 60 h, irrespective of the route and murine strain. The average time-to-death, however, varied according to the administration route. For the intraperitoneal and intranasal routes these were similar with little variation between strains. The average times to death by the intraperitoneal route were 3 days, 2-3 days and 3 days for Porton, Balb/C and NIH/S, respectively. When administered by the intranasal route, the average times were 3 days, 2 days and 2-3 days. After subcutaneous challenge the times to death were 5-6 days, 4 days and 4 days, respectively (Russell et al., 1995).
• Side Effects: Mice immunized with the Plague vaccine, USP, developed local lesions at the site of the injection of the challenge including grey/white caseation of the focal lymphatic glands. This lesion would either track to other sites between the dermis and the muscle fascia or penetrate the muscle fascia forming a lesion between bundles of muscles. In some cases the lesions ulcerated and began healing (Russell et al., 1995).
• Challenge Protocol: The mice were challenged either s.c. or i.n. with Y. pestis GB strain and observed for 14 days.
• Efficacy: Plague vaccine, USP, induced protection against a subcutaneous challenge with Y.pestis strain GB. Two series of trials were conducted, both with the challenge strain growing exponentially. In the first trial mice were challenged with multiples of the MLD up to 50 000. There was 100% mortality in the control groups, treated with 5 and 50 MLDs, l/5 deaths in the vaccinated group challenged with 50 MLD, and 2/5 deaths when challenged with the highest dose. In the second trial, in which challenge doses were extended from 4200 to 4 200 000 MLD, between 40 and 60% of the animals died at each dose. Thus USP conferred protection against the s.c. challenge with 5 x l0^3 MLD of Y. pestis strain GB. However, this protection does not extend to challenge with high multiplicities of the infectious dose (Russell et al., 1995).