HERD Immunity? you mean a measles party? There is more to immunity than just Antibodies.
Why do you think Adults are getting childhood diseases? because their artificial immunity is wearing off, Booster time!! Do you really think our immune system needs a reminder?
Vaccine FAILURES are Well Documented, yet . . .

The AMA reported that “measles is increasingly becoming a disease of adults,” admitting that the demographics of this typically childhood disease are shifting to adults as vaccines wear off too early.[4] Almost all childhood illnesses are deemed much more dangerous in adults. Mumps and rubella are also being reported to be shifting toward older children and adults. During epidemics, measles has been known to attack nearly 100% vaccinated populations.[5] Similar statistics have been seen during mumps outbreaks. Rubella, which is very innocuous in childhood, is now threatening women of child-bearing age (the very group the shot was supposed to protect to prevent birth defects).[6] After nearly 55 years of medical hoopla, the Centers for Disease Control reports that pertussis is at the highest levels since 1967. [7] Constant (and profitable) boosters are now the government’s only solution for shortsightedly disrupting the natural course of childhood illnesses. Yet, the policymakers are never apologetic.

1. Science, March 26, 1977.

2. FDA workshop to review warnings, use instructions, and precautionary information [on vaccines], July 18, 1992.

3. Mendelsohn, Dr. Robert, How to Raise a Healthy Child...In Spite of Your Doctor, p.218.

4. Preventing Emerging Infectious Diseases: A Strategy for the 21st Century. (U.S. Dept. of Health & Human Services/CDC publication), Oct. 1998, p.22.

Measles Childhood disease!

California Measles Surveillance Update February 13, 2015
Since December 2014, there have been 113 confirmed measles cases reported in California residents.
Table 3. Age distribution of confirmed measles cases
http://www.cdph.ca.gov/HealthInfo/discond/Documents/Measles_update_2_13_2015_public.pdf

Age (years) Total Percent
Age <1 - 11 total = 10%
Ages 1-4 , 15 total = 13%
Ages 5-19 , 20 total = 18%
Age 20 > 67 got it = 59%

As you can see from the above chart the majority of those that got the “childhood disease” measles were not children. They were adults. Why?

More on ALL THIS
http://www.vaclib.org/basic/Koren-measles.pdf