Since antibodies gradually fade over time, most vaccines are given in multiple doses over an extended time. If you get hepatitis B immunization, you get 3 shots over 6 months. Most childhood vaccines are spread out over years. The current shingles vaccine is given as two shots, 2 to 6 months apart.
The mRNA vaccines were tested (and thus approved for use) using a relatively tight time frame, 3 or 4 weeks apart. I am sure this was because of the urgency of getting these vaccines tested and available. Indirect evidence from Britain, where the second shot was deliberately delayed to permit as many first shots as possible, suggests a longer interval gives even better protection.
Real world evidence shows that these vaccines are extremely effective, even against the Delta variant, with 80%+ prevention of infection and 90%+ prevention of hospitalization out to 6 months. The one group where evidence strongly suggests a third dose is needed are those with depressed immune systems, such as transplant patients.
A recent study found that among nursing home residents, who got the vaccine early, the protection gradually waned, falling as low as 53% 8 months after vaccination. Another, looking at New York residents, found that protection against infection dropped from 92 to 80%, though protection against hospitalization remained over 90%.
A very small recent study, done by Pfizer, found a marked increase in antibodies when a booster dose was given 8-9 months after the second original vaccine.
SO:
1.Yes, a booster is probably a good idea.
2.If you have a depressed immune system, you should get this as soon as possible.
3.If you are otherwise healthy, there is no need to panic and seek one out before you are due.
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