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Vaccines prevent severe disease, but omicron drove more breakthrough cases in Arizona

new report from the state health department shows COVID-19 vaccines continue to save lives in Arizona. But the highly contagious omicron variant drove a steep increase in breakthrough cases in the state in December and January.

More than 35% of Arizona’s COVID-19 cases in December were among people who were fully vaccinated — more than double the rate of breakthrough cases the state recorded in August. Graphs in the report appear to show the rate of breakthrough cases increasing even more in the beginning of January, though the report does not provide complete data for that month. 

Medical experts believe the number of COVID-19  cases the state officially reports is likely a vast undercount as an increasing number of people use at-home tests, and many people may become infected without ever getting tested. 

The state's COVID-19 hospitalization and death records are likely more accurate. And Jessica Rigler, with the Arizona Department of Health Services said hospitalization and death records continue to show that the vaccines provide very strong protection against the virus, in spite of rising breakthrough infections.

“When omicron began to spike here in Arizona, people who were unvaccinated were 24 times more likely to be hospitalized and about 58 times more likely to die from COVID-19, compared to those who were fully vaccinated,” Rigler said. 

The report compares only Arizonans who have never had any vaccine for COVID-19 and those who have been fully vaccinated. It does not include data for people who were partially vaccinated. The report also does not differentiate between someone who has been fully vaccinated and someone who has been fully vaccinated with a booster dose. Numerous sources, including the  Centers for Disease Control and Prevention, report booster doses provide a far higher level of immunity than initial doses alone. 

The state's report shows Arizona's highest rates of death and hospitalization as a percentage of total population by age group, are among unvaccinated people over the age of 65. 

"We strongly urge all Arizonans to get vaccinated if they haven't already, those who are 5 and older. This is particularly important for individuals over the age of 65," Rigler said. “For those who are over 65, the importance of being vaccinated and receiving a booster dose are critical to preventing severe disease from COVID."

The  CDC reports about 84% of Arizonans 65 and older are fully vaccinated — the tenth-lowest rate among states. And about 57% of fully vaccinated Arizonans in that age group have had a booster dose — the fifth-lowest rate among states. 

→  Arizona COVID-19 hospitalizations may not peak until mid-February

Katherine Davis-Young is a senior field correspondent reporting on a variety of issues, including public health and climate change.