I'm looking for the name of a (English-language) book read in the 1970s
The main plot focused on an alien scout-ship crash-landing and killing half of a town.
The (crashed) aliens take the identities of people who were killed in the crash and live among the population for around 20 years and have children who don't know they're are aliens.
The aliens were taking injections (shots) to keep themselves looking human.
They ultimately defend the town from an (impending) alien invasion by committing acts of sabotage. The fleet is getting close.
The lead looked uncannily like an old lady's son who died in vietnam.