Can you cite where you are getting that bolded part? I am not certain but I am pretty sure that is factually wrong. I believe the basic understanding is that NORMALLY they occur within 3 months.
However, we have never developed a vaccine so fast so their is nothing normal about the situation we're in. We are in uncharted territory, basically, with the covid vaccine, like it or not. I don't blame people for being cautious with it.
No, there really is no plausible biological mechanism for a vaccine to somehow cause a side effect that appears
de novo months or years down the road. Unlike medications, which are taken continuously and can cause dose-related toxicity through chronic exposures, the actual material of vaccine contents is broken down and completely excreted from the body within days, if not sooner. The lingering influence of the vaccine does not occur because of retained vaccine material, but in the selection of a population of B and T cells that respond specifically to the virus or bacteria in question. Very rarely, an error in the regulation of the immune response can produce B or T cells that react against the person's own cells... basically, an autoimmune response. Because the self-antigens on people's cells don't ever change, once the vaccine induced-immune cells mature (within days to weeks of receiving the vaccine), they will either autoreact soon thereafter or never. Fortunately, autoimmune disease from vaccination is extremely rare, or else vaccination would not be a viable strategy for disease prevention.
Some side effects can linger for years, like the sequelae of Guillan-Barre syndrome, or the exceptionally rare case of polio from the live vaccine (no longer administered in countries where polio is effectively erradicated). But the inciting condition occurs soon after the vaccination.
Just in case I was remembering something wrong, or there's a notable reaction that hadn't come up in my medical education or nearly two decades of practice, including supervision of mass-vaccine distribution in the army, I reviewed some articles from
UptoDate and
Micromedex (excellent medical reference sites, but both behind paywalls) on various vaccines, such as TDaP, MMR, influenza, meningococcus, HPV and some atypical vaccines that most in the US never receive (typhoid, anthrax, yellow fever). The longest delayed side effect I could find was Guillan-Barre syndrome that is most associated with some varieties of seasonal influenza vaccines (but the disease is far more common after an actual bout of influenza). As I quoted previously, the longest suspected delay of a case of this condition after vaccination was just under three months, although it usually appears much sooner.
Finally, it should be stated once again why we were able to develop the COVID vaccines so quickly:
1) The mRNA technology already existed, and it was very easy to adapt this to the spike protein of SARS-CoV-2.
2) Governments around the world provided up-front funding, thus the pharmaceutical companies didn't need to pause between the research stages to acquire financing for the next stages.
3) The trials reached their predetermined clinical endpoints much faster than expected, mainly because the placebo groups accumulated positive cases very quickly. This is the silver lining in the black cloud of the raging out of control pandemic.
The trials conducted the same standard safety observation periods that any vaccine would receive.