4.11: Tracking the Spread of Covid-19

During the Covid-19 pandemic, caused by the virus SARS-CoV-2, many of us have heard a lot of different measurements or rates discussed in the news. These measurements (or indicators) are tools that epidemiologists use to understand how a disease is spreading and how to control or prevent it.

Indicators

In addition to counting cases,measuring the incidence rate, and calculating the mortality rate, there are some other indicators used in addressing an epidemic like Covid-19. Here are a few of the main ones:

Reproduction number or R0, pronounced "R nought":This is a calculation of how fast the disease is spreading. R0 measures how many new people catch the disease from a single case, on average. If the R0 is 1, that means for each person who gets the disease spreads it to only one person -- this would make the rate of disease stay flat, not going up or down. When the R0 is higher than 1, the rate of disease (morbidity rate) will increase, and when R0 is less than one, the rate will decrease. While every disease has a theoretical R0 that's fixed (for how many people would catch the disease from a single case in a population with 0 immunity), in practice, it changes over time and from place to place. For example, in a location where people wear masks routinely, the R0 will be lower than in a place where people seldom wear masks, because masks reduce the transmission of disease.

Case Fatality Rate (CFR): This is a rate that calculates the proportion of people who die, out of all the confirmed cases. This rate is not FIXED for a given disease, since a lot depends on the age of the infected person, what healthcare is available, and other factors that change. In the last 18 months, we have greatly reduced the case fatality rate of Covid-19 through improved treatment for the disease -- though it still has a much higher case fatality rate than the flu or many other common diseases. A related rate is the infection fatality rate (IFR), which is the proportion of people who die out of all the cases, period -- including those that are not diagnosed or confirmed. Since we cannot know the real number of people infected at any given time, we have to use estimates for the IFR -- but we can use actual counts for the CFR. When an outbreak is new and less in known about how to treat a disease, fatality rates are likely to be higher. Also, when an epidemic surges, as with the Delta variant surge in summer/fall 2021, fatality rates go up, because healthcare systems get overwhelmed.

Positivity Rate This is the proportion of positive tests out of all the tests given to detect a disease. For example, on October 5, 2020 in San Francisco, the positivity rate was 0.5%, and on January 6, 2021, the positivity rate was 10 times higher, at 5.0%. The rate on September 9, 2021 was 3%. To get a reliable positivity rate, you have to do A LOT of testing -- so you can be sure the tests are reflecting what's really going on in the population.

Hospitalization Rate The hospitalization rate represents the percent of the population in a given area that are hospitalized with a positive Covid-19 test. It varies a lot by age and by location. There is a lot of attention right now on hospitalization rates, both because some hospitals are overwhelmed right now, and because everyone is looking to see if the vaccines are working to prevent hospitalizations and death. This graphic of what's happening with hospitalization rates during the Delta surge gives you a sense of how well they are working.

CDC's focus switching from case rates to hospitalization rates in assessing risks

Through the first two years of the pandemic, a lot of focus has been placed on case rates -- how many people are testing positive? Some people with positive test results (cases) will become ill, while others will not get sick, or have only mild symptoms.

With an increasing proportion of the population having some level of immunity now, either through past infection or through vaccination, or both, along with the relatively milder symptoms of the Omicron variant, the CDC has switched their focus to give more weight to hospitalization rates when they rate communities for risk. This is more similar to how other diseases are monitored -- we don't test everyone for flu, for example; we gauge how bad the flu season is by hospitalizations and deaths.

There are pros and cons of this approach! As covid has about a 2 week lag time between infection and hospitalization, by the time the hospitalization rate goes up, you can assume that a lot of people are already infected. Monitoring cases and test positivity lets you respond more quickly to changes in the prevalence of the disease. But the focus on hospitalization is a better measure of the impact of the disease and the needs of health systems for support. What do you think is the measure that is most important to focus on?

ICU bed capacity This rate is calculated as a way to monitor how ready the hospitals are to receive Covid-19 patients who need intensive care. When the ICU bed capacity is low, hospitals may repurpose other rooms in the hospital to attend patients, transfer patients to other hospitals, or open temporary "field" hospitals. The ICU bed capacity isn't just a count of beds -- it also depends on having nurses and other personnel available to staff the ICU. And keep in mind, some of those beds are needed for patients with other diseases or injuries, besides Covid.

In 2020, you also may have heard about "flattening the curve". Flattening the curve does not make the epidemic go away, but it does save lives, by slowing things down. That way, the hospitals aren't so overwhelmed and more people can get the medical attention they need. The idea is to keep the number of people who are sick low enough that the hospital capacity can respond adequately.

In 2021, with the more-contagious Delta variant of the SARS-CoV-2 virus that causes Covid, we again saw hospitals in some parts of the country running out of ICU beds. This is primarily occurring in areas of the country with low rates of vaccination. While the Covid vaccines do not entirely prevent a person from getting Covid (infections), they do a very good job at keeping people out of the hospital and reducing the mortality rate.

In the 2022 Omicron wave of covid, we saw many more people getting infected -- the R0 went up -- but with a much lower rate of hospitalization and death, because the virus itself is less virulent and because most people had acquired some degree of immunity -- either because they were vaccinated, or because they had covid before, or both.

When we take precautions -- or mitigate the risks -- through social distancing, masks, good ventilation, and no crowds, a lot of people still get sick, but they are more likely to live through it. Why? Because the healthcare capacity -- hospitals, nurses, doctors, medications, personal protective equipment, syringes, etc. -- is more available when there are fewer patients needing care at once.

Photo credit: Photo by Trnava University on Unsplash