since the cotton strain 19 vaccine is a live attenuated vaccine the development of immunity will just follow the course of a natural infection. I have following questions:
1. Do the vaccinated heifers shed the vaccine strain?
2. If they do, what is the risk for incontact animals and human handlers?
Taking into consideration that only female (not male) cattle are vaccinated, and that seronegative non-pregnant females are the preferred candidates for vaccination, the shedding of Brucella abortus S19 in milk of vaccinated cattle is largely dependent on the vaccinal S/C dose and the animal age at vaccination.
Vaccinal S/C dose: There used to be the full dose of 40 to 120 × 10˄9 in 5 mL that is no longer applied. Currently, 2 doses exist, viz. the reduced dose of 3 to 10 × 10˄9 in 2 mL and a more reduced dose of 3 to 8 × 10˄8 in 2 mL. The latter more reduced dose is specially suitable for adult and biannual vaccination in case of high risk areas. Low doses tend to result in less persistent serological titers and less shedding than higher doses. Generally speaking, reduced doses result only in limited shedding of the vaccine. Pregnant cows may occasionally abort with shedding in milk and vaginal mucous. Vaccination of infected animals tends to reduce the shedding rate of Brucella field strain.
Age of vaccination: The best age is 3 to 6 up to 8 months old. The older the animal, the more likely to develop persistent serological titers and to shed the vaccine as compared to young animals.
Regarding the potential infection risk of S19 in humans, accidental injection with the vaccine can lead to brucellosis. Human-to-human transmission is very very very rare (via sexual contact and breastfeeding). I’d like to indicate that there is a human vaccine against brucellosis licensed in Russia known as strain 19-BA that is very much similar to S19 of animals. S19 usually causes mild illness in humans that may require partial or no antibiotic treatment according to the severity of the disease and monitoring of body temperature.
Brucella abortus strain 19 vaccine:
The most widely used vaccine for the prevention of brucellosis in cattle is the Brucella abortus S19 vaccine, which remains the reference vaccine to which any other vaccines are compared. It is used as a live vaccine and is normally given to female calves aged between 3 and 6 months as a single subcutaneous dose of 5-8 × 1010 viable organisms. A reduced dose of from 3 × 108 to 3 × 109 organisms can be administered subcutaneously to adult cattle, but some animals will develop persistent antibody titres and may abort and excrete the vaccine strain in the milk. Alternatively, it can be administered to cattle of any age as either one or two doses of 5 × 109 viable organisms, given by the conjunctival route; this produces protection without a persistent antibody response and reduces the risks of abortion and excretion in milk when vaccinating adult cattle. Brucella abortus S19 vaccine induces good immunity to moderate challenge by virulent organisms
The Brucella abortus S19 strain is a reference vaccine and used to control and eradicate brucellosis from bovines by adopting the policy of (1) Test and slaughter/ segregation/culling, (2) Vaccination of negative population and (3) Strict sanitary measures, by many developed countries. However, situation is different in developing and economically poor countries as they cannot afford slaughter/culling of positive population and also difficult to maintain sanitary conditions on farms. This result in endemic situation and only option for such countries is to adopt vaccination with S19 vaccine. Now the question is that the S19 is a live vaccine and can cause infection to human beings. Other queries are related to shedding of S19 vaccine from vaccinated animals in milk etc. On these issues researchers/experts above have appropriately answered to the questions you raised and I am in agreement with them with following caveats:
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