Methodology
The Post adjusted coronavirus rates for cases, deaths and hospitalization over time by combining CDC data on
cases,
hospitalizations and
vaccinations. The Post used a rolling seven-day average of daily cases, deaths and hospitalization. For vaccinations, The Post used the number of people who had received at least one shot as of each date.
For events like coronavirus infection, rates are usually calculated by dividing the number of cases by the number of people in the population. For example, if there are 12 cases among a population of 100 people, the rate would be 12 people per 100. The Post reduced the denominator to exclude most vaccinated people. So if 20 people got vaccinated, that would mean there were 12 cases out of the remaining 80 unvaccinated people, for an adjusted rate of 15 cases per 100 people.
Vaccination is not perfect in preventing infections, however, so The Post did not subtract the entire population of vaccinated people. Data shows the Pfizer-BioNTech and Moderna vaccines are
about 90 percent effective in preventing cases among people who have received the shot. Cases among vaccinated people are called
breakthrough cases. To be conservative, The Post estimated that up to 15 percent of the vaccinated population could still be infected.
So, in the example above, instead of removing all 20 vaccinated people, The Post removed 17. That would leave 12 cases among 83 people, for an adjusted rate of 14.5 cases per 100 people.
Using data released by the CDC’s Walensky, National Institute of Allergy and Infectious Diseases director Anthony S. Fauci and several states concerning effectiveness of vaccines for preventing hospitalization and death, The Post subtracted 90 percent of the vaccinated population from the denominator for hospitalization and 98 percent of the vaccinated population from the denominator for death rates.
The Post calculated the adjusted rates of cases, deaths and hospitalization for the nation and each state since the start of vaccination in December. Coronavirus case and death rates released by states are sometimes subject to time lags. State also sometimes review older cases and issue updated figures that reflect a backlog of old cases rather than a surge on that day.