Analysis: the numbers of newly identified cases and deaths allow us understand and learn where we are in the course of the outbreak
Every night, the Government releases statistics to provide updates on the Covid-19 pandemic in Ireland. This data includes the number of newly identified cases, the number of deaths that day, the total number of cases and the total deaths. But what do these numbers actually tell us? And what can we learn from them about the state of the outbreak?
Why didn't the numbers of new cases decline once restrictions started?
When the schools closed and movement restrictions were implemented, we did not see an immediate drop in the number of cases. In fact, they continued to increase. This does not mean that the measures are not working: it has to do with when the individuals were exposed.
When an individual is exposed to Covid-19, the virus is in an incubation period for an average of 5 days, though this has been found to range from two to 14 days. The incubation period is the time between when an individual was exposed and when they begin to show symptoms, which means that someone who begins to show symptoms today was likely exposed to the virus five days ago.
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Additionally, there will be a number of days from the initial onset of symptoms before someone seeks out a test and still further delays due to test availability. Someone identified as a case today was likely infected about two weeks ago. Therefore, any decrease or slowing down in the number of cases due to an intervention will not be appear in the data until weeks after the interventions are implemented.
How will we able to tell if we are reaching the peak of the outbreak?
We can learn about how the pandemic is progressing in Ireland from the number of new cases identified today especially when this number is combined with the total number of cases. These two numbers can give us an idea of how rapidly the disease is spreading by finding the percent increase in the number of cases which can help us to determine if we are reaching the peak of the outbreak. The peak of the outbreak is the point when we have the maximum number of individuals infected at any one time. After this, the total number of active cases should begin to decrease.
The peak is the maximum point of the infection curve, the curve that shows the total number of active cases (those infected and not yet recovered) over time. This can be found when the change in the total number of active cases switches from increasing to decreasing. As we reach the peak of the outbreak, the rate of change of the number of cases should start to decrease and move closer to zero (even though the total number of active cases will still be increasing until we reach the peak).
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How does the peak relate to flattening the curve?
The peak of an outbreak is not a predetermined number nor does it occur at a predetermined time. The idea behind flattening the curve is to reduce the overall size of the peak. Since the peak is where we will have the maximum number of infected individuals at any one time, this means it’s the time when we will have the maximum demand on our healthcare system. If we expect a certain number of COVID-19 patients to be severe, the more cases there are at a given time the more severe cases there will be.
Where does "herd immunity" come into all of this?
When we reach the peak and the number of cases starts to drop does that mean that we are out of the woods? Unfortunately no as there is a possibility for the virus to have a resurgence and a second peak to occur. This could happen if we were so successful in following the movement restrictions and social distancing that we have not reached herd immunity.
Herd immunity is a concept that is often discussed and was a key point in the UK’s initial response to the pandemic. The idea is that there is a critical number of individuals or percentage of the population that need to be immune to interrupt the transmission of the disease within a population. For a well known disease like measles, small pox or polio, we can - and in some cases have - reached herd immunity through vaccination.
With no vaccine, we would have to let Covid-19 spread until the right proportion of individuals in the society were infected and become immune, something which could mean many unnecessary deaths. Once herd immunity is reached, there are not enough susceptible individuals within the population for widespread transmission of the disease. Small local clusters might still occur but they will not persist.
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Herd immunity is directly related to the basic reproduction number (R0) of the disease. R0 is the average number of cases an infectious individual is likely to infect in a completely susceptible population. The higher the R0, the higher the percent of the population that will need to be immune to reach herd immunity. With an R0 of 2, the lower bound of the WHO estimates of R0 for Covid-19, herd immunity would be reached if 50% of the population are immune. A more infectious disease such as measles, which has an R0 of 12-18, would need to see between 92 and 94% of the population immune to reach herd immunity.
If we have not reached herd immunity then there will likely be a resurgence of Covid-19 once all restrictions are lifted. This is why it is important to have a plan to slowly lift restrictions so that any clusters of Covid-19 that do erupt can be quickly identified and controlled.
The number of deaths and the Case Fatality Ratio
What can the number of deaths tell us about the outbreak? The total number of deaths combined with the total number of cases is used to find the case fatality ratio (CFR). This is the number that is often quoted when discussing how many deaths will occur from the pandemic and has been shown to vary across countries. The CFR is determined by dividing the number of death by the total number of cases.
However, it’s important to remember that this is actually the total number of identified cases. The number of identified cases is likely understated because those with mild or asymptomatic cases and still others that will not meet the testing criteria will not be counted. Because it is only the identified cases that are used as the denominator, the CFR will be an overestimation of the actual fatality rate of the disease. A country that does more widespread testing such as Germany will have a lower CFR compared to a country that does less testing such as the United States strictly because the denominator for a country with widespread testing will be larger. This makes it difficult to compare CFRs across countries, especially ones with different testing policies.
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From RTÉ Six One News on March 3rd, WHO estimate that the mortality rate from Covid-19 was around 3.4%
Additionally, the CFR will change as the outbreak progresses. Early on in an outbreak, fewer cases will have come to a conclusion, either recovery or death, thus the CFR will not include the cases that are active now but will result in death in the numerator. This means that the CFR might also be underestimating in some way the actual fatality of the disease and again make it difficult to compare across countries that are at different stages of the pandemic.
The mortality rate and the pandemic
Looking at the mortality rate is another way to see how fatal a disease is. This is a measure of the number of deaths in a defined population for a period of time and is often expressed per 100,000 individuals. A cause specific mortality rate can be found to determine the mortality rate for a specific cause of death such as Covid-19. As of April 15th, Ireland has a CFR of 3.5% and a mortality rate of 9.15 deaths per 100,000 people. The CFR gives an idea of how deadly the virus is if you catch it and the mortality rate gives an idea of how likely you are to catch the virus and then die from it.
The mortality rate is not affected by the testing policies of a country so does not have that potential overestimation. However, it is important to consider the population structure and where a country is in the course of the outbreak when comparing mortality rates for different countries. For Covid-19, a country that has an older population will likely have a higher mortality rate than a country with younger demographics. Similarly, when calculating the mortality in the middle of the pandemic, a country that has reached its peak should have a higher mortality rate than a country that has not yet reached its peak.
It is important to understand where some of the numbers and terms come from in order to be able to be as informed as we can be
Although they can be easily misunderstood or misinterpreted, the numbers of newly identified cases and the number of deaths can provide us with important information to understand where we are in the course of the outbreak. It is, however, important to understand where some of the numbers and terms come from in order to be able to be as informed as we can be from these numbers and understand where Ireland is and how we are faring compared to other countries in the world.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
