Is april or march too late to get your flu shot ?

Not so. Flu activity is not limited to Winter. Certain flu strains have caused large outbreak even in summer, e.g., H1N1 strain in 2009.

By CDC website: Ideally, everyone should be vaccinated by the end of October. However, even if you are not able to get vaccinated until November or later, vaccination is still recommended because flu most commonly peaks in February and significant activity can continue into May.