Fauci following the science in 2004: "The most potent vaccination is getting infected yourself"

Asked if a person's who has recently had the flu should get a flu shot, Fauci responded in 2004:

"If she's had the flu for 14 days, she's as protected as anybody could be because the best vaccination is to get infected yourself. If she really has the flu, she definitely doesn't need a flu vaccine. She doesn't need it because the most potent vaccination is getting infected yourself."

This is entirely correct. Naturally acquired immunity is robust (which means that it also protects quite well against variants of pathogens) and probably lifelong.

You are well protected as long as your current immune state is strong. If you have a chronic sleep deficit, chronic stress, eat and drink lots of junk food, have low vitamin D, take immune suppressing medication, etc. you are no longer protected well regardless of immune memory.

If you get sick in such a weak immune state, this doesn't mean that naturally acquired immunity does not work, but that your immune state is weak. Your immune system still recognizes pathogens reasonably well, but is so weak that it's unable to put up an adequate response. Vaccination in this case is unlikely to do you much good because a vaccinated weak immune system is still a weak immune system.

The solution in this case is not vaccination, but to improve your immune state through optimal lifestyle and diet among other things, such as improved environment (e..g adequate ventilation and optimal humidity).

Here are some relevant scientific articles:

ARTICLE 1: SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls | Nature
https://www.nature.com/articles/s41586-020-2550-z

People who were exposed to SARS-CoV-1 in 2003 had excellent T-cell immunity against SARS-CoV-2 in 2020. This shows that immunity is both very robust and durable.

ARTICLE 2: Neutralizing antibodies derived from the B cells of 1918 influenza pandemic survivors | Nature
https://www.nature.com/articles/nature07231

People who were alive during the Spanish flu of 1918 were still immune almost 90 years later in 2008.

ARTICLE 3: Influenza and obesity: its odd relationship and the lessons for COVID-19 pandemic
https://pubmed.ncbi.nlm.nih.gov/32249357/

The obese are more easily infected, more & longer infectious, have increased risk of more virulent strains.

"Conclusions: Due to prolonged viral shedding, quarantine in obese subjects should likely be longer than normal weight individuals."