Although California and Arizona are geographically next to each other, both states have had very different experiences with the coronavirus.

That's because unique circumstances within both states have affected the progression of SARS-CoV-2 coronavirus infections within each during the pandemic.

That observation is driven home when we compare the reporting of newly confirmed COVID-19 cases in both states. Here, because California doesn't make the same kind of high quality data available to the public that Arizona's Department of Health Services does, we're turning to Johns Hopkins CSSE COVID-19 Data as our data source for both states. Using this data will allow for a more apples-to-apples comparison for applying the back calculation technique we've developed in analyzing Arizona's experience with the coronavirus pandemic.

That technique involves identifying turning points that changed the trajectory of newly reported cases following events that changed the rate of incidence of new infections. These turning points begin appearing in the rolling seven-day moving averages for newly reported cases some 9 to 11 days after the events that changed the viral infections rate of incidence, which corresponds with when 95% of all infections following an initial exposure event have developed. This process identifies a specific window of time in which contemporary news reports involving large gatherings may be reviewed to identify any "superspreader" events that prompted the change in the rate at which the coronavirus spreads.

That's the background - let's look at what happened in California during the last four months of 2020.

In this chart, the vertical dashed lines indicate when the trajectory of newly reported cases definitively broke from the seven day moving average trend immediately preceding it, as indicated by the heavy blue line. The orange-shaded vertical bands indicate the period some 9-11 days earlier in which the events triggering the change in trend would have occurred. We've labeled five events in the chart with letters from A through E, the following section describes the corresponding events and their subsequent effect on California's reported incidence of COVID-19 cases.

Major Milestones for COVID-19 in California

Event A: L.A. Lakers win NBA Championship

After several weeks of following a flat-line trend averaging 3,200 new cases per day, the incidence of COVID-19 cases in California notched up to average roughly 4,400 new cases per day beginning on 22 October 2020, with the increase largely contained within Los Angeles County. This change occured 10 days following the L.A. Lakers NBA championship on 12 October 2020, which saw large crowds in L.A. celebrating the victory. Compared to what happened later however, the Lakers victory had a comparatively low impact.

Event B: L.A. Dodgers win World Series

The Dodgers victory over the Tampa Bay Rays in the 2020 World Series was the real spark that ignited California's late surge of new coronavirus cases. Here, we see the change in trend with the number of cases rising definitively above their post-Lakers championship high on 6 November 2020. Ticking the calendar backward by 9 days puts us at 28 October 2020, which is when the Dodgers became baseball's world champions. Much larger crowds in L.A. celebrated the event, pointing to baseball's much larger fan base in the region. Following the Dodgers victory, Los Angeles County led California for growth in the number of new cases, with the surrounding counties that make up L.A.'s greater metropolitan area following suit.

Event C: "Emergency Brake" Restrictions Go Into Effect

The next significant change in trajectory took hold beginning on 25 November 2020 (Thanksgiving Day), 9 days after California Governor Newsom imposed new restrictions on the operations of California businesses. The restrictions appear to have been somewhat successful in sharply slowing the rate of increase that began with Dodgers' world series win, but proved to be short lived.

Event D: A Clumsy Curfew and Governor Newsom's Loss of Credibility

On 2 December 2020, the number of new COVID-19 cases in California began increasing faster than they had before. Tracking significant events backwards, we find Governor Newsom's announced month-long curfew was the trigger for the increase, although it occurred 12 days earlier, on 23 November 2020.

That's because the curfew announcement sparked a popular backlash against Governor Newsom, with large political protests beginning the following day, which falls within the expected 9-11 day window. The protests came as public outrage spiked following the publication of photos of Governor Newsom disregarding his COVID-19 rules at a dinner held at the exclusive French Laundry restaurant less than two weeks earlier.

Governor Newsom was far from the only Calfornian official to shred their own credibility through outright hypocrisy during this time. On 4 December 2020, Governor Newsom announced a new stay-at-home order for California residents, while local governments announced they would ban outdoor dining, but instead of reducing the state's upward COVID-19 trajectory as might be expected in the period from 13 through 15 December 2020, the number of cases rose sharply as many small restaurant owners and their customers began protesting the officials' apparent determination to destroy their businesses. These actions were further undermined when California's top public health official confirmed the state and local governments' new restrictions lacked any support from scientific data indicating they would be effective in slowing the rate of COVID-19 infections.

Update 22 January 2021: Infectious disease experts have identified the ban on outdoor dining as a primary cause of California's winter surge of excess coronavirus cases, hospitalizations, and deaths.

Event E: California's ICUs Fill Up with COVID Cases and Christmas Travel

It's not until 22 December 2020 that we see any reversal in California's COVID trends, which corresponds to events that transpired from 11 through 13 December 2020. Here, we think the key event that altered California's COVID-19 case trajectory were reports of ICUs in the state nearing their capacity. We think these stories combined with the caution that Californians planning to travel to celebrate Christmas with their families began adopting in advance of the holiday, where increased social distancing contributed to reversing the surge provoked by Governor Newsom's hypocrisy.

Meanwhile, During the Same Time in Arizona...

Arizona's coronavirus experience is very different from California's. The following chart tracks four significant events in the state's coronavirus experience in the period from 1 September 2020 through 31 December 2020, which we've labeled as Events F through I.

Note the very different vertical scale in the chart as compared to California's! The following section describes the four events identified on the chart:

Major Milestones for COVID-19 in Arizona

Event F: High risk businesses reopen

Following its early summer surge in cases, Arizona's number of new COVID-19 cases had bottomed out with roughly 550 new cases reported each day through September. The number of new cases however began rising slowing after 3 October 2020, which points to events that occurred from 22 through 24 September 2020 as leading to the increase. There was no large single event in this case, but this period marks when most businesses believed to have high exposure risk for spreading COVID-19 infections were allowed to reopen in the state, though with some restrictions on their operations.

Events G and H: 2020 Political Campaign Events

2020 was unusual in that Arizona was a swing state for the U.S. presidential election. Unlike in California, both Republicans and Democrats held significant events across the state on two weekends preceding the 3 November 2020 election. The changes in trend some 9-11 days after these two weekends suggests these were superspreader events that boosted the rate of growth of new COVID-19 infections within Arizona, regardless of political party affiliation.

Event I: Arizona's ICUs Fill Up with COVID Cases and Christmas Travel

That upward trend in new cases continued until 19 December 2020, after which the number of new cases has begun decreasing. As with California, this change in trend follows widespread news reports from 9-11 days earlier indicating that ICUs in Arizona were nearing capacity.

Although they are adjacent to each other, the events that have contributed to the fall and winter surge in coronavirus cases in Arizona and California are very different from one another. At the same time, the two states have adopted very different strategies for coping with the coronavirus pandemic. Generally, Arizonans have been more free to engage in commerce and other activities than Californians have, with little apparent downside. Californians however have faced an increasing level of heavy-handed restrictions on their activities through these four months, with little apparent benefit. There are many lessons to be taken from this tale of two states and the coronavirus.

Bonus Update: 8 January 2021: Since we mentioned Arizona has higher quality COVID-19 data available, we'll add charts showing its data for newly confirmed cases by sample collection date, daily ICU bed usage, deaths by death certificate date, and new hospital admissions, covering the period from 3 March 2020 through 5 January 2021. Although the lag from exposure to change in trend for each chart is different, the back calculation method for each confirms the identification of significant events we discussed for Arizona above.

Of these charts, only the chart showing Arizona's ICU Bed Usage is fully current. Data for the other three charts are incomplete, where the most recent three weeks shown will be subject to revision during the next few weeks, especially for the most recent dates indicated on the charts.