eizzo@adirondackdailyenterprise.com Saranac Lake doctor David Johnson is seen here Wednesday, April 7 inside the gymnasium at Keene Central School, where the Essex County Health Department hosted a COVID-19 vaccine clinic. (News photo — Elizabeth Izzo) KEENE VALLEY — Standing in the gymnasium at Keene Central School, Dr. David Johnson readied supplies at his station at a COVID-19 vaccine clinic on Wednesday, April 7. Johnson, who ran a private practice in Saranac Lake, retired in July after 34 years of caring for patients. He’s not done yet. He’s one of about 100 medical professionals who have signed up to serve as part of Essex County’s Medical Reserve Corps in response to the coronavirus pandemic.