Turn on the TV news or watch a news website, and you’ll see charts, graphs, and dashboards that supposedly indicate the latest with COVID-19: statistics revealing the number of tests, cases, hospitalizations, and deaths, along with where they happened and whether they are rising or falling.
Different stories are told depending on the board. But one thing is certain: these indicators lag behind the actions we take, or not, in COVID-19. As researchers focused on public health, we can tell you that a fully accurate, real-time snapshot of the virus’s progress is not possible.
Some don’t get tested
There are many reasons for this. Here’s one: Diagnostic test data is incomplete. Someone infected with COVID-19 must first come into contact with the virus through the air or (less likely) from environmental surfaces. Symptoms show up two to 14 days later. But at least 40% of those infected will never manifest symptoms or show symptoms so mild that they don’t even suspect they have COVID-19. Therefore, they may never be tested, which means that they will not appear in the total number of tests or in the total number of cases.
Another example: Due to unavailability of tests, a widespread problem in the US Since the start of the pandemic, not everyone who needs to be tested is tested.
And another: the tests themselves are not perfect. Up to a third who test negative may actually be infected. This happens because they are tested before they have a sufficient viral load for detection. Or perhaps sampling is not adequate. Or maybe the test itself simply failed.
AP Photo / Lynne Sladky
Case numbers do not tell the full story
That is why problems arise when we use case numbers to determine disease levels in a community. The case counts actually reflect what was happening in a community weeks before. Four weeks, for example, could elapse between the time a person is exposed to the virus and when it is reported as a case. Even the best test results often take a week to report to public health authorities, and take longer to appear on dashboards. Some delayed test results can take ten days or more.
[[[[Get facts about the coronavirus and the latest research. Sign up for The Conversation newsletter.]Other factors impact metrics. Laboratory results, often published in batches, can introduce artificial variations in case numbers. Someone who did the test two days ago and then got a result right away, could be added the same day as someone who was sick two weeks ago but whose test results were delayed. To smooth these variations, it is helpful to look at a seven-day case average.
Hospitalization is a clearer measure to assess the level of community illness. Those who are seriously ill, in most cases, will be hospitalized whether or not they have been previously examined. The data suggests that about one in five infected people is hospitalized. People appear to be fine during the first week, with more life-threatening symptoms in the second. That means that hospitalizations represent exposures that occurred three to four weeks earlier.
Again, a seven-day moving average equals artificial variations. There is a caveat to this: Although hospitalization is a helpful measure, only about 20% of infected people need it. That means that only hospitalization numbers underestimate the number of people infected and the age groups they represent.
States vary by cause of death
Death numbers are also not a reliable indicator. In some states, to count as a COVID-19 death, the deceased must have had a test that reported positive. In other states, probable cases are reported.
As doctors learn how to better treat COVID-19, death rates are declining. Deaths, the longest indicator of delay, reflect infected people six to eight weeks earlier. When comparing one region with another, deaths are best expressed as rates, a proportion of deaths per population.
Another problem: News reports don’t always make a clear distinction between diagnostic tests, which show whether you currently have COVID-19, and antibody tests, which show whether you have had it in the past, and now harbor antibodies that can fight it. So far, however, antibody tests have not provided a useful picture of who has been infected and who has not. Once that happens, it could provide researchers and clinicians with some pointers on the spread of the virus.
Although control panels are ubiquitous on television, none of these frequently used indicators they present is perfect. Still, taken together, they provide a reasonable approximation of COVID-19 transmission in communities. But as authorities make decisions, they must keep in mind that the numbers are weeks old.
what does this mean to you? Understanding these limitations can help you understand your risk. We are still in the midst of a pandemic that is not under control. Being educated will help us all become part of tomorrow’s lagging indicators.
