Which Covid Data Numbers Really Matter

All data is better than nothing. But some data are better than others. JL

Caroline Nyce reports in The Atlantic:

Today's cases represent people who were exposed a week or two ago. Weekly trends provide a more accurate picture than any single day's count. Watch out for data lags: Each state has their own reporting cadence: some report daily, and some take weekends off. As a result, numbers can have a day-of-the-week effect. Hospitalizations are more stable than case numbers. Test positivity rates are easily skewed: factors like viral prevalence (how many infections there are), testing utilization (who is getting tested), and varying units (the number of people tested or raw tests) can make this metric very difficult to interpret

Pandemic data can be difficult to parse and weigh. Below, two experts offer five tips for improving your COVID-19 data literacy.

In order to understand this pandemic, you need to understand the data. And that’s not always easy: Each day, a variety of local, state, and federal sources release a firehose of statistics.

Which numbers should you pay attention to and which should you absorb with a grain of salt? I asked two experts from The COVID Tracking Project, Jessica Malaty Rivera and Peter Walker, for some advice on how to get smarter about COVID-19 data. They offered five tips.

1. Remember that today’s cases represent people who were exposed a week or two ago.

Symptoms can take several days to appear. Likewise, it takes time for a patient to get tested, get their results back, and for those results to appear in the dataset. Keep in mind that what you see in daily COVID-19 data is oftentimes a look into the recent past.

2. Weekly trends provide a better picture than any single day’s count.

Using the 7-day averages can help account for lags in COVID-19 data. Trends are generally more reliable, when you’re looking at both local and national data.

3. Watch out for data lags.

Each state has their own reporting cadence: some report daily, and some take weekends off. As a result, numbers can have a day-of-the-week effect, which happens when backlogged data shows up a few days later.

4. Hospitalizations are generally more stable than case numbers.

Although many hospitalization metrics are inconsistently reported at the state level, national COVID-19 hospitalizations have been one of the best measures of what is actually happening on the ground throughout the pandemic’s ebbs and surges this year. It’s as close to “real-time” data as we can get.

5. Test positivity rate is easily skewed.

On the surface, the calculation for test positivity (also known as percent positive) is simple: Divide the number of positive tests by the total number of tests, in a select period of time, then multiply the result by 100.

Unfortunately, factors like viral prevalence (how many infections there are), testing utilization (who is getting tested), and varying units (the number of people tested or raw tests) can make this metric very difficult to interpret—and even harder to use for public health policy.

One question, answered: If my immune system is unusually overactive or weakened, can I still get the vaccine? “In short, yes,” our staff writer Katherine J. Wu reports:

The C.D.C. currently advises that people with weakened immune systems or autoimmune conditions can still get their COVID-19 vaccines. But there’s limited data on whether the currently available vaccines, made by Moderna and Pfizer, will confer the same degree of protection to people in these groups, or elicit the same side effects. Because vaccines are designed to train the body’s immune cells to fight off infections, they may not work as well in people taking immunosuppressants, or who are living with HIV, for example. People with overactive immune systems, on the other hand, might harbor trigger-happy immune cells that get unnecessarily riled up in the presence of a vaccine, sparking a flare-up of uncomfortable or perhaps even dangerous symptoms. Conditions that affect the immune system are not well understood.

But health workers have had a lot of experience treating these conditions, including in the context of vaccinations. And many people who have altered immune responses may be at a higher risk for developing a severe case of COVID-19 if infected by the coronavirus. In that light, experts tell me, the benefits of getting vaccinated outweigh the risks of passing up a shot, as long as people stay in contact with their doctors.