I have read many claims from (German) politicians COVID-19 vaccines only work for 6 to 9 month. It is being used to argue that vaccinations should be mandatory in Germany and Austria every 6 or 9 months.
A Nov 25, 2021 Zeit article [German] describes an EU proposal that would make EU-wide vaccination certificate last for only 9 months, unless additional booster vaccinations are given. (English article discussing the same proposals).
A May 15 2021 Frankfurter Rundschau quotes a couple of health experts as saying that the length of time the protection lasts before waning is unknown, but one assumes it will be around 6 months.
An August 27 NY Post article explains Dr ANthony Fauci and US President Biden discussed whether a booster shot should be required as early as 5 months or as late 8 months after the second shot.
What does the evidence show?
I think an answer should differentiate at least the following aspects:
Probability in dependence of time after first complete COVID-19 vaccination to:
- transmit the virus to others
- get ill with mild COVID symptoms
- get ill with severe COVID symptoms
- need to go to the intensive care unit because of COVID
- to die because of COVID
- to develop a serious "long COVID" syndrome
The aspects should also be differentiated between the available vaccines on the market and also differentiated per age group and underlying health conditions. One should also distinguish the virus variants.
For me it is at least conceivable that the aspects above are not strongly correlated, for example that the probability to transmit the virus to others may increase significantly after 6 or 9 month, but the probability for severe symptoms may not.
What does the evidence show? In particular if you want to decide if it is proportionate to make the vaccination mandatory every 6 or 9 month, I think you need a more differentiated picture about the date than only claiming that the vaccine "doesn't work" anymore after this or that timespan.