It is a highly successful immunogen. That is why one doesn’t get flu or colds in the summer. Flu vaccine is less effective at preventing flu. Sometimes it is the wrong vaccine due to a bad guess as to what type is coming our way. Around here, among the people I know who get the shots several also get the flu every year. If you get off on that needle then have at it. I will have fewer colds and flu than you will through the years. So far it is none in 9 years, ditto my family and friends for however long they have been doing the vitamin D.
The idea of vitamin D being a successful immunogen against influena is questionable, but, albeit, promising. But, there are too many other factors in question when it comes to the seasonality of the flu. And, you certainly can catch the flu or a cold in the height of summer. Ever heard of summer colds?
The CDC certainly has access to all the studies and data they need. If you choose not to give their findings any credibility, that’s your decision. We can just agree to disagree.