The fatal shootings at two health centres are thought to have been provoked by Nigerian Muslim leaders claiming that vaccines cause polio and sterility.
While religious opposition has slowed the adoption of polio vaccination programmes in Nigeria, this is the first time that health workers delivering the vaccine there have been killed.
Due to the religious opposition to vaccination, Nigeria is one of only three counties in the world where polio remains endemic.
Vaccines are among the greatest breakthroughs of modern medical science, but in recent years global vaccination programmes have been dogged by ideological objections and hostile myths.
Last week, an influential Islamic cleric in Nigeria claimed that polio is caused by vaccination. In Pakistan, where polio health workers have also been recently killed, the Taliban has claimed that immunisation causes sterility.
Extremist anti-vaccine movements have grown in the US in recent years, fuelled by political opposition to public health strategies.
In the UK, anti-vaccine prejudice may have driven the ‘MMR panic’, sparked by a research paper that was later exposed as fraudulent (though the Daily Mail still supports it).
Polio is an incurable viral disease that can cause permanent paralysis, and used to be a major cause of disability in Europe. It mostly affects young children. Repeated doses of polio vaccine can confer lifelong protection against the virus.