In a land of free speech the writer of the anti-vaccine letter is entitled to have his letter published. But given the import of the topic your newspaper has responsibility to ensure that you report the story and not just provide a platform for those who wish to disseminate their version of the truth. Publishing such a dramatic overstatement of dire impacts is irresponsible on your part. Otherwise you perpetuate the current plethora of “false facts” that continually are given headline coverage.
This is topic of major and expensive public health policy that effects all citizens and particularly in a city with such an elderly population, who are given a universal message to get a flu shot.
Prudence would suggest that before publishing such a letter you should have done two things. First approach a neutral academic source who might provide factual scientific information to address the validity of technical claims of the writer. And then second you should provide equal space for our public health officials to provide a rejoinder.
This topic is too important to the well-being of the community to simply allow unsubstantiated info to be disseminated in our newspaper without some balance and then let readers make their own personal choices.