What are the downsides of getting the vaccination

The short version: the evidence for waning is real but inconsistent, and the benefit of timing is modest.

The UK study from 2011-12 showed the clearest waning pattern: protection against H3N2 dropped from 53% for those vaccinated less than three months before exposure to 12% for those vaccinated three months or more before [1]. That is a 41 percentage point decline. The same study found no waning for influenza B, which stayed at 92% protection regardless of timing [1]. This tells you waning depends on the strain and the vaccine match that year.

For a fit 69-year-old man, the practical question is when flu season peaks in your region. In most of the UK and Europe, that is January through March. Getting vaccinated in late October or early November puts peak antibody levels right in that window. Vaccinating in August or September risks having protection fade by February [1]. Vaccinating in December or January leaves you unprotected during the early part of the season.

The confidence intervals on these estimates are wide. The 53% figure had a 95% confidence interval from 0% to 78%, meaning the true protection could be anywhere from zero to quite good [1]. The 12% figure had a confidence interval from -31% to 41%, meaning it is not statistically different from zero [1]. So the evidence supports delaying vaccination until close to the season start, but the benefit of that timing is modest and depends on the year's match.

My call: for a fit 69-year-old man, aim for late October or early November, about 2-4 weeks before local flu season typically peaks. This gives the best chance of protection when flu is circulating. Confidence: moderate.