The suggested citation information (but not APA format) for Healthy People 2020 main site is:
Healthy People 2020 [Internet]. Washington, DC: U.S. Department of Health and Human Services, Office of Disease Prevention and Health Promotion [cited [Date URL was accessed]]. Available from: [Specific URL].
What this tells us is that the author is a Group Author. A URL to the page should be included. If it is a page that will change over time, then a date should be included.
Following APA 7th edition, if I were to use this https://www.healthypeople.gov/2020/topics-objectives/topic/immunization-and-infectious-diseases it would look like this in APA style 7th edition:
Office of Disease Prevention and Health Promotion (n.d.). Immunization and Infectious Diseases. https://www.healthypeople.gov/2020/topics-objectives/topic/immunization-and-infectious-diseases
HHS.gov, SurgeonGeneral.gov, HealthCare.gov or Flu.gov Web pages
You may cite content from an HHS website by using the date, page title, and the URL.
With Author names:
Cullen, K. A., Ambrose, B. K., Gentzke, A. S., Apelberg, B. J., Jamal, A., & King, B. A. (2018). Notes from the Field: Use of Electronic Cigarettes and Any Tobacco Product Among Middle and High School Students — United States, 2011–2018. MMWR. Morbidity and Mortality Weekly Report, 67(45), 1276–1277. https://doi.org/10.15585/mmwr.mm6745a5
No author names, organization instead:
Centers for Disease Control and Prevention. (1999). Control of infectious diseases. MMWR. Morbidity and Mortality Weekly Report, 48(29), 621–629. http://www.ncbi.nlm.nih.gov/pubmed/10458535
MMWR is a publication series, once published in print. Treat it like a journal with the author being the Centers for Disease Control and Prevention