COVID And The Cold Front

October 30, 2021

A year ago this week, Los Angeles’ shocking COVID-19 winter wave began. Although few realized it at the time, the rise came in new cases the week after the average temperature dropped.

It was the first indicator of a terrible reality: colder weather is forcing people indoors, where coronavirus, like the winter flu, is more easily transmitted. This was confirmed when more than 775,000 cases of coronavirus were diagnosed and over 13,700 residents of the county died of COVID-19 from December to February.

Although much is different now – starting with the fact that 71% of eligible county residents are fully vaccinated and 79.5% of Angelenos have received at least one shot – mercury has begun to decline. And the region has just seen its first weekly increase in cases since early August, according to data from the Los Angeles County Department of Public Health.

Recently, the director of public health warned Dr. Barbara Ferrer on the risks that lie ahead.

“We still need to lower the number of new cases so we go into the fall and winter seasons in the best possible way to avoid the devastation we endured last year,” Ferrer said on September 25. “We typically see respiratory viruses flourish with the colder weather and when we move activities indoors.”

Low case period

For much of September last year and the first half of October, the Los Angeles County’s weekly number of new coronavirus cases remained relatively low and stable, around 7,000-7,800 each week. For most of this period, the climate was warm, with daytime highs in the 80s or 90s and evening lows in the mid-60s, according to data on

Over the next few months, a drop in temperatures one week was often followed by an increase in cases the next, suggesting that people may have gathered indoors more often. For example, the average daily high temperature in the week was 18-24. October 75 degrees, down from 90 the week before.

Next week, 25-31. October, the county registered 8,995 new cases, a 24% increase from the 90-degree week.

Weekly new coronavirus cases in Los Angeles County, Oct. 4-Dec. 5, 2020

The increase then accelerated, the change in weather often followed quickly by more people getting the virus. From the 8.-14. November, the average daily high was 67, and the average low was 48. This compared to an average high of 78 and an average low of 57 the week before.

Next week, from the 15th-21st. November, nearly 26,000 cases were diagnosed, representing a one-week increase of 53%. Transmission and positivity rates rose rapidly (also due to Halloween and Thanksgiving gatherings). From 6.-12. December, 74,424 new cases were registered.

The gift

Los Angeles is not automatically in a repeat of last winter. In addition to widespread vaccinations, there are many mask mandates. Awareness of security protocols generally remains high.

Yet last week the county recorded its first increase in new cases in more than two and a half months. It was a modest increase, only 5.4%, to 7,505 new infections.

Weekly cases of coronavirus and temperatures in Los Angeles, September 5-Oct. 23, 2021

But the increase coincided with a change in the weather. A heat wave that had daily highs in the 80s and 90s for the first few days of the month was followed by a period from the 7th-13th. October, when the heights for the most part were no more than 73, and the evening lows of five of these days fell in the 50s.

The possibility of a setback has chosen, and health leaders are calling for action. When he received a Moderna booster shot on Wednesday, Gov. Gavin Newsom called on eligible Californians to get their next dose.

“It is imperative that we all keep our immunity strong in preparation for the winter cold, flu and now the COVID season, especially for those at increased risk,” he said in a prepared statement.

It might come at the right time. Although it was unusually hot today and Thursday, the Accuweather forecast requires much cooler temperatures from Halloween.

How we did it: We examined coronavirus data from the Los Angeles County Department of Public Health. We also researched archived daily high and low temperatures on

Interested in our data or have a question? Send us an email at

Crosstown is a non-profit local news organization based in Los Angeles. They use data to provide important insights to communities to help people make their neighborhoods safer, healthier and more connected. Sign up for their free weekly newsletter on to get neighborhood crime, air quality and traffic data delivered to your inbox.

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