Some bacteria can evade efforts to vaccinate against them by wearing a new disguise, researchers say.
A study, published in Nature Genetics, tracked how pneumococcus bacteria responded to the introduction of a vaccine in the US in 2000.
Doctors said the evasion would make some vaccines less successful in the long term.
An updated pneumococcus vaccine is already in use.
Vaccines train the immune system to attack something unique to an infection. In the case of tetanus, it results in the body making antibodies which target the toxin produced.
Dr Rory Bowden, one of the researchers from the University of Oxford, told the BBC: "There are plenty of vaccines out there that look stable and continue to work because they target bacteria or viruses that are not changing."
Pneumococcus bacteria, however, comes in more than 90 varieties or serotypes. Each variety looks different to the immune system so would each need separate vaccines.