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Natural immunity might be slowing COVID-19, but Arizona is far from herd immunity
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Natural immunity might be slowing COVID-19, but Arizona is far from herd immunity

Iris Delfakis, oncology nurse navigator for the Arizona Cancer Center, looks to other nurses as she waits to receive a Pfizer COVID-19 vaccine from nurse Cristina Torres.

at Banner-University Medicine North, 3838 N. Campbell Ave., in Tucson, Ariz. on Dec. 17, 2020. The first round of Pfizer’s COVID-19 vaccines are given in Pima County went to healthcare workers at Banner-University Medical Center and Tucson Medical Center.

In recent weeks, Arizona and Pima County have seen a record surge in the rate of positive serology tests. These tests look for antibodies, which are specific proteins that fight off infections, to determine if someone has already been infected.

At the same time, COVID-19 cases recently dipped from one week to the next. And the percentage of positive diagnostic tests also fell for two consecutive weeks. Both metrics, however, still remain at high levels.

At face value, this is encouraging news. It suggests that relatively more people have been recovering from the virus while fewer people contracted it.

“I think that that’s a good sign,” said Dr. Francisco Garcia, Pima County’s chief medical officer. “And I’m hoping that we continue in that vein. At the same time that we are vaccinating.”

Experts like Garcia, however, are cautious when interpreting the rate of positive serology tests.

“It’s probably accurate in the sense that ... more Arizonans have been infected and recovered. I think that’s an obvious statement that serology doesn’t necessarily need to help us answer,” said Dr. Joe Gerald, an associate professor with the University of Arizona’s College of Public Health.

What experts really want to know is the percentage of Arizonans who are immune to COVID-19 and won’t become infected again, whether they achieved immunity from a vaccine or by recovering from an infection.

A slower spread

This tally would help them know when the state is approaching herd immunity.

The R0 value, which is pronounced “R-naught,” suggests that herd immunity could be reached when 60-65% of people are immune, Gerald said. Garcia’s target is a little higher, coming in at 75%.

The R0 value is the average number of people an infected person infects if no one is immune. For COVID-19, that number is about 2.5, Gerald said.

We are nowhere near such widespread herd immunity that COVID-19 stops spreading, Gerald said.

But at half the percentage needed to reach herd immunity, we could start to see the virus spread at a slower pace, he said. As more people become immune, each infected person starts to run into more people he or she can’t infect.

Theoretically, serology tests could help us count how many people are immune by detecting antibodies. Scientists at the Centers for Disease Control and Prevention are still researching how much protection antibodies grant patients from getting infected again, but reinfections remain rare.

At this point, however, serology testing can’t effectively do the job, Gerald said.

“The scale of serology testing is just really, really tiny compared to the size of Arizona,” he said.

This leads to a huge risk of sampling bias, he said. Those showing up to get tested aren’t a good representation of the population as a whole. People get tested for specific reasons. Many think they have been infected or know they have, which skews the rate of positive test results.

Statewide, the rate of positive serology tests spiked from a recent low of 9% in the last week of October to 38% in the second week of January, according to the Arizona Department of Health Services’ chart of test results, as of Friday. Countywide, it rose from 6% to 31% over the same period.

Gerald estimates that at most 30% of all Arizonans have natural immunity after having contracted the virus.

When you add that number to the less than 1% of people who have been fully vaccinated statewide, he said, we’re getting closer to half of what we need to reach herd immunity.

By other estimates, the state has even fewer people who are naturally immune. Tim Lant, a mathematical epidemiologist at Arizona State University, said most estimates are between 20% to 25% have natural immunity.

75% immunity possible by summer

In Pima County, 63,084 total vaccine doses have been administered, according to the ADHS, as of Friday. Statewide, a total of 380,764 vaccine doses have been administered. Many who have received the vaccine have not received the full two doses.

That combination of natural immunity and vaccine-induced immunity may be enough to tap the breaks on viral spread and level out cases, Gerald said. But he estimates that 75% of the population won’t be immune until the summer.

“So we actually are beginning to get in the neighborhood where natural immunity could be slowing the cases down,” he said. “How confident am I in that? Not confident enough to say that’s definitely why cases turned down. But I think it is beginning to be a plausible reason why case counts may not go much higher than they are now.”

Across the state, week-to-week cases decreased by 25% in the second week of January, dropping to 48,422, while diagnostic tests fell by 13%. The percentage of positive diagnostic tests also decreased from 22% to 20%.

Countywide, week-to-week cases decreased by 22% to 6,841 in the second week of January, while diagnostic tests fell by 12%. And the percentage of positive diagnostic tests fell from 22% to 19%.

While natural immunity may play a role in slowing the virus, continuing infections will lead to even more deaths and hospitalizations. And hospitals don’t have much more room to help people recover.

Only 8% of inpatient beds and 9% of ICU beds were available on Thursday across Arizona, according to ADHS’s chart of hospital bed usage and availability, as of Friday. Statewide bed capacity hasn’t changed much in the past week.

New coronavirus hospitalizations increased statewide by 9% to 3,762 in the first week of January, which broke yet another record. In Pima County, coronavirus hospitalizations dipped by 1% to 405, according to data published on Wednesday from the Pima County Health Department.

The Star typically waits two weeks to interpret COVID-19 death and hospitalization data to account for data reporting delays. But sometimes the data is delayed even longer, and these numbers could increase even more as more data are potentially backfilled. Over the past week, for example, about 260 more statewide hospitalizations were reported for the first week of January.

Across the state, COVID-19 deaths rose by 2% to 840 in the first week of January, setting yet another record, according to the ADHS’s chart of deaths by date.

In Pima County, 98 people died from COVID-19 in the first week of January, which was down 22% from the week before, according to the data published by the county’s Health Department.

“We’re nowhere near (herd immunity),” Lant said. “It’s not a viable strategy... unless you want to burn through the health-care system for another three months like this.”

Contact reporter Alex Devoid at or 573-4417.

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Data/Investigative Reporter

Alex has been with the Star since June 2019. He previously wrote about the environment for the Arizona Republic and he's a graduate of the University of Arizona.

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