There is no evidence that the "vaccine only works for a couple of months".
The vaccine has continued to show efficacy against hospitalization and death (severe covid) even as there has been some waning in infection immunity over a period of multiple months (more than 2 or a couple months though). See e.g. https://www.cdc.gov/mmwr/volumes/70/wr/mm7049a2.htm "During February 1–September 30, 2021, mRNA vaccine effectiveness in preventing COVID-19–associated hospitalizations among U.S. veterans ≥120 days after receipt of the second dose was 86% for Moderna and 75% for Pfizer-BioNTech vaccines"
Future covid strains may be more or less deadly - data coming in on Omicron indicates it's more deadly than original COVID but perhaps slightly less deadly than delta. Still quite dangerous to an unvaccinated or immune naive individual.
WTF - then why are "booster shots" needed? In my country they are moving towards recommending them after three months already.
I don't have a English language resource ready, but this article in a renowned German science magazine shows protection against severe illness is drastically reduced after six months for the current vaccines: https://www.spektrum.de/news/wie-lange-schuetzt-der-impfstof...
For example from the chart, after 6 months the protection from Biontec is only 30% of the protection in the beginning.
“ The effectiveness against severe illness seems to remain high through 9 months, although not for men, older frail individuals, and individuals with comorbidities. This strengthens the evidence-based rationale for administration of a third booster dose.”
Possible difference between the Swedish study and the CDC - time period comparison of 4 months (120 days) vs 9 months (270 days) - differences in Swedish population or statistical errors.
Just eyeballing CDC data on who is dying of coronavirus it’s very hard to see how any statistical analysis could show the vaccine not “working” - the smaller unvaccinated population is making up a majority of hospitalizations and deaths in the USA, and most vaccinated persons haven’t received a booster in USA.
It seems likely that future Covid strains will be less deadly, too, just as the flu has years that are more or less deadly than others.
Also, isn't a fraction of the population immune to begin with?