How effective is influenza immunisation for healthy adults?

How effective is influenza immunisation for healthy adults?

Answer: 

It takes up to two weeks after immunisation for the body to start developing protection against influenza. When the influenza strains in the vaccine are a good match to the influenza strains circulating in the community, around  half to two-thirds of healthy adults aged under 65 years of age who received the vaccine will be protected against influenza. The immunisation can also protect almost two-thirds of these adults from being unwell enough with influenza to need hospital care. 

Influenza immunisation doesn’t protect against the common cold or other viruses and diseases in circulation. Influenza immunisation does not protect against coronavirus infections.