So many things wrong with your statement:
- The flu shot isn't "likely to make you sick". The vaccine is either an inactivated form of the virus (it's been killed by the body will increase immunity) or from a single gene of the virus which is not capable of making someone sick.
- A person's immunity to flu viruses declines over time, so it's not a matter of "I've had it once, so I'll be fine".
- "The flu vaccine isn't effective" - Vaccines have been found to reduce the need to visit a doctor up to 60%. This also means that it is extremely good at minimising the level of spread. Those that do get sick have reduced likelihood of severe symptoms and quicker recovery.
Want to know why they push for the flu vaccine but don't make it compulsory:
Now you're going to add the fact that there is a new virus for which there is no vaccine, and the resulting disease also attacks the respiratory system. I think the NRL would be smart to expect players to have the vaccine.