The Trouble With the Case Curve During the Holidays
The familiar curve chart that shows new Covid-19 cases will be disrupted by testing and data reporting interruptions just as people prepare for another holiday season in the United States. Last year, the national case curve showed pronounced declines after Thanksgiving and Christmas that did not reflect real decreases in new infections.
The impact of holidays may be even more noticeable this time around, as illustrated by the recent Labor Day holiday, because states are reporting data less consistently than they did a year ago. In response, we’re updating the averaging methodology that turns the daily case and death data we report into a curve chart.
How Our New Averaging Method Affects the U.S. Case Curve
New case average
Previous case average
200,000 reported cases
Declines in testing and reporting disruptions over the holiday season toward the end of 2020 caused artificial declines in known cases.
Our new average helps to correct some of the problems caused by irregular reporting of cases around holidays.
Christmas and
New Year
150,000
Thanksgiving
Labor Day
100,000
50,000
0
Sept. 2020
Nov.
Jan. 2021
March
May
July
Sept.
Nov.
200,000 reported cases
Declines in testing and reporting disruptions caused artifical declines in the case curve.
Our new average helps to correct some of the problems caused by irregular reporting of cases around holidays.
Christmas and
New Year
100,000
Labor Day
Thanksgiving
0
Sept. 2020
Nov.
Jan. 2021
March
May
July
Sept.
Nov.
200,000
reported
cases
Declines in testing and reporting disruptions caused artifical declines in the case curve.
Our new average helps to correct some of the problems caused by irregular reporting of cases around holidays.
100,000
Christmas and
New Year
Labor Day
Thanksgiving
0
Sept. 2020
Jan. 2021
May
Sept.
200,000
reported
cases
Declines in testing and reporting disruptions caused artifical declines in the case curve.
Our new average helps correct some of the problems caused by irregular reporting around holidays.
100,000
Christmas
and
New Year
Labor Day
Thanksgiving
0
Sept. 2020
Jan. 2021
May
Sept.
This change will help show infection trends, including the 14-day percent change in cases and deaths, more accurately around holidays and other periods of atypical data reporting, such as when officials cannot release new data for several days because of technical or staffing issues. Unfortunately, it will not help to address artificial drops in infections resulting from declines in reported testing, like those we saw around the winter holidays last year.
What to Expect From Data During the Holidays
Case counts remain an important part of tracking how the virus is spreading, but the curve for the United States is the result of a multistep process that is sensitive to interruptions. If testing sites have limited hours or appointments for the holidays, people may not get tested or they might use home tests that are not reported to officials. If labs or government offices are closed or short-staffed, it takes longer to record test results and share that information with the public.
Stephen Kissler, a postdoctoral fellow in immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, said these fluctuations show up in flu data as well. Usually, there is a surge in precautionary testing before holidays, which can drive positivity rates down, and the decline is followed by an increase in cases since people would have been less likely to get tested over the holiday period, even if they had symptoms.
“It will probably take a good week or two before things settle back down and we can see what actually happened over Thanksgiving,” Mr. Kissler said.
The decentralized public health system in the U.S. means that thousands of state and local health officials are independently responsible for reporting data related to coronavirus testing and cases.
In 2020, New Jersey was among the states where a large increase after the holidays reflected both a data lag and holiday-related spread of the virus. Dr. Edward Lifshitz, medical director of the Communicable Disease Service at the New Jersey Department of Health, said the state expects a similar pattern this year, though hopefully not a major increase in new cases.
“The overall trend is important, but it’s kind of like watching the stock market,” Dr. Lifshitz said. “You don’t want to look at it every single day because you can go crazy trying to figure out why random variabilities are happening.”
The average number of cases reported each day declined after both Thanksgiving and Christmas, but these drops corresponded to decreases in the number of tests performed, meaning that more cases were likely going uncounted. Once tests returned to previous levels, cases quickly climbed again to their peak in early January. Those deceiving decreases were not reflected in the chart showing the number of people hospitalized with Covid-19 during the holiday season.
New reported cases by day
Tests by day
Hospitalizations
250,000 cases
1,500,000 tests
200,000
100,000 hospitalized
150,000
1,000,000
Christmas
and New Year
Christmas
and New Year
Christmas
and New Year
100,000
50,000
500,000
Thanksgiving
Thanksgiving
Thanksgiving
Nov. 2020
Dec.
Jan. 2021
Feb.
Nov. 2020
Dec.
Jan. 2021
Feb.
Nov. 2020
Dec.
Jan. 2021
Feb.
New reported cases by day
Tests by day
Hospitalizations
250,000 cases
1,500,000
tests
200,000
100,000
hospitalized
150,000
1,000,000
Christmas
and New Year
Christmas
and New Year
Christmas
and New Year
100,000
50,000
500,000
Thanksgiving
Thanksgiving
Thanksgiving
Nov. 2020
Dec.
Jan. 2021
Feb.
Nov. 2020
Dec.
Jan. 2021
Feb.
Nov. 2020
Dec.
Jan. 2021
Feb.
New reported cases by day
250,000 cases
200,000
150,000
Christmas
and New Year
100,000
Thanksgiving
Nov. 2020
Dec.
Jan. 2021
Feb.
Tests by day
1,500,000 tests
1,000,000
Christmas
and New Year
500,000
Thanksgiving
Nov. 2020
Dec.
Jan. 2021
Feb.
Hospitalizations
100,000 hospitalized
Christmas
and New Year
50,000
Thanksgiving
Nov. 2020
Dec.
Jan. 2021
Feb.
The number of people hospitalized with Covid is a lagging indicator of community infection rates, but is less sensitive to holiday data interruptions because data from hospitals is reported directly to the Department of Health and Human Services. And if a hospital fails to report for a single day, the same number of Covid-19 patients from the previous day can be easily carried forward.
Why We’re Updating Our Averaging Method
As the pandemic evolved in 2021, we updated our tracker pages to emphasize rolling averages instead of daily counts. In our data, cases and deaths are included on the date that state or local health officials add them to the cumulative total.
Compared to November 2020, when most states provided daily updates, the majority of states now announce updates only on weekdays or even less frequently. Many states report zero cases on weekends and then catch up with a large one-day total on Mondays, a tally which reflects the state’s process for reporting infections rather than an actual change in case trends.
To reduce volatility caused by holiday reporting obscuring the actual trends, we’re switching to a rolling average with a more flexible time range. The previous method displayed the average of the day and the previous six days of data. In the new method, if the seven-day window includes days with no data reported, the period is extended to older days until it includes seven days of reported data.
The figures for some days may count as more than one day’s worth of reported data. For instance, data reported on a Monday following two weekend days where no data was issued will count as three days of data. Under the new method, the count from a day following nonreporting days is always averaged over that day and the nonreporting days that precede it.
How the New Average Works
The Times now uses an average calculation that always includes at least seven days of reported data in each day’s average. The average figure for each day can now span multiple calendar days.
Daily new cases
Previous case
average
New case
average
Labor
Day
F
S
S
M
T
W
T
F
S
S
M
T
W
T
F
S
S
M
T
W
T
F
S
S
M
T
W
T
F
Most weeks, the new method continues to use the same seven-day average.
Over Labor Day weekend, the new average wouldn’t have changed when the state reported no data.
The artificial spike in the previous average, which resulted from adding two weekends’ worth of data, is corrected by averaging cases over a 10-day period.
Daily new cases
Previous case
average
New case
average
Labor
Day
W
T
F
S
S
M
T
W
T
F
S
S
M
T
W
Over Labor Day weekend, the new average wouldn’t have changed when the state reported no data.
The artificial spike in the previous average, which resulted from adding two weekends’ worth of data, is corrected by averaging cases over a 10-day period.
The rolling average may include data for more than seven days when the data is not regularly reported during that period. Certain days with anomalous total case or death figures are excluded from the average or have a portion of their cases and deaths removed from the average calculation when they correspond to data backlogs.
For national case and death counts in the U.S., the average is calculated from the sum of the average number of cases and deaths reported each day in all states and territories. This average may not match the average that results from the daily total of U.S. cases and deaths. This is to account for irregularly timed case and death reports at the state level.
For more about how this data is reported and published, see our data F.A.Q.
During the weeks leading up to Thanksgiving last year, cases were increasing in all regions of the U.S. and vaccines were not yet available. This year, with varying vaccination rates and case trends across the country, the picture is more complicated. Comparing case trends with other metrics such as test positivity rates and hospitalizations at the local level can provide a better sense of how the virus is spreading.
Experts recommend taking proven precautions such as wearing masks, gathering outdoors or in well-ventilated spaces, and making plans to get tested before and after holiday events.
“It does make sense for people to take precautions and recognize that the pandemic is not over,” said Julia Raifman, an assistant professor of public health at Boston University. “But I think a lot of the future looks like navigating uncertainty and balancing risks and rewards, because we also think it’s important to gather with family. People can certainly enjoy the gathering and take steps to reduce the risks.”