Last month, we shared a new way the Centers for Disease Control and Prevention (CDC) looks at the effectiveness of COVID-19 vaccines in light of rare breakthrough cases. 

This approach examines the difference between unvaccinated and fully vaccinated individuals when it comes to COVID-19 cases and deaths. It uses rates of an outcome among each group to develop a ratio of how much more likely one group is to have that outcome. The public health term for this comparison is incidence rate ratio (IRR).

Now the CDC has created a website allowing you to see the incidence rate ratio in action for cases and deaths. It shows that during August, unvaccinated individuals were 6.1 times more likely than fully vaccinated individuals to test positive for COVID-19 and 11.3 times more likely than fully vaccinated individuals to die from COVID-19. 

The site allows you to adjust the curve by vaccine type and age group. Every view tells the same story: Whichever vaccine a person received, whatever their age, those who are fully vaccinated are much less likely to get COVID-19 and die from COVID-19 than those who aren’t vaccinated. You also can click a link at left to see COVID-19-associated hospitalizations by vaccination status, and this curve shows the same robust protection for those who are vaccinated.

Given the recent news about vaccinated members of the Arizona Cardinals organization testing positive for COVID-19, it’s natural for people to ask about the effectiveness of vaccines. That’s also the case with today’s passing of former Secretary of State Colin Powell from COVID-19, though Powell, who was vaccinated, had a compromised immune system because of multiple myeloma.

No vaccine can provide 100% protection, but it’s clear from these CDC presentations and other sources that COVID-19 vaccines continue to offer strong protection against infection, severe illness, and death. That is, if you get a breakthrough case, your illness is in nearly all cases going to be much less severe than someone who isn’t fully vaccinated, subject to factors such as age and health conditions. 

We’re frequently asked for the number of breakthrough cases and the percentage of hospitalizations and deaths among those who are fully vaccinated and unvaccinated. For August, 87.1% of those hospitalized for COVID-19 weren’t fully vaccinated, while 87.5% of deaths were among individuals who weren’t fully vaccinated. 

But that view of hospitalizations and deaths presents an incomplete view of the situation. Here’s why: As more and more people get vaccinated and there are fewer cases, hospitalizations, and deaths as a result, those who are fully vaccinated will necessarily make up a larger percentage of cases, hospitalizations, and deaths. That’s why ADHS will be moving toward incidence rate ratio as a better way to understand risks for those who are and aren’t vaccinated. 

After getting down in the weeds of public health data, I’ll return to today’s subject: the clear value of COVID-19 vaccines, as told by the incidence rate ratio. Vaccines continue doing their job of reducing the risk of severe illness and death from COVID-19. That’s why we’ll continue recommending these steps to protect yourself and others: 

  • Get Vaccinated: We strongly urge you to roll up your sleeve and get vaccinated today at one of the hundreds of providers around Arizona. The vaccine is our best line of defense against COVID-19. It protects you, those you love, and your community. 
  • Get a Booster: If you are among those recommended for a COVID-19 booster, please get one. At this time, boosters are recommended for certain groups who received the Pfizer vaccine, but boosters are likely coming for the Moderna and Johnson & Johnson/Janssen vaccines. 
  • Follow Mitigation Strategies: Whether you are vaccinated or not, wear a mask and physically distance when you are indoors around people you don’t live with. You’ll find other ways to protect yourself at