Why was the Spanish Flu so deadly?

The 1918 Spanish Flu pandemic was the deadliest outbreak of the flu virus in history.

Flu outbreak had a long history. Why was the 1918 flu deadliest?

It was just after World War I (WWI). Does the high death toll have anything to do with the war? I have been wondering for a long time.

Recently, I read a book. After WWI, many Germans died of starvation due to the blockade of the allied forces. Germans were not the only ones to be punished. Many countries were among the losers. Even on the side of the winners, there were many to be educated. Many Irish sought independence from Britain. Many colonial people wanted to rid of their masters. I don't know much about history. From what happened to Germans, many people probably were not well nourished. Poorly nourished people succumbed to infections easily, even minor infections.

How about this pandemic?  Vaccine is useless in stopping the spread of viruses and causes many harmful effects. Many people died from vaccines and other so called health measures.

History and information are always controlled by powerful people. But from little is known, it seems that powerful people are much more deadly than flu viruses and coronaviruses.

The following link is from Robert Roos about Pfizer vaccine.

https://youtu.be/Qo8Ra9NWPRY