Researchers are developing a methane vaccine for livestock to ‘help reduce agricultural emissions.’ The vaccine aims to stimulate the animals to produce antibodies against the methane-producing microbes in their stomachs.

In other words, this 'vaccine' would attempt to train the animals' bodies to fight against the methane-producing microbes in their stomachs.

They claim this would produce less methane and release it into the atmosphere.

https://www.rnz.co.nz/news/national/490956/millions-invested-to-progress-methane-vaccine-for-livestock