Reporting on covid-19 typically
tends to include a variety of statistics. Beyond the number of cases, there are infection
rates, case fatality rates etc. Today we explain what some of those
numbers and statistics mean.
Positivity tells us the proportion of those tested who are actually confirmed to have been infected with the disease. As of 23rd May, Ghana Health Service reports that they have tested 197,194 people out of which 6,808 have tested positive leading to a positivity rate of 3.44%. The positivity rate can tell us a lot or nothing at all about what the disease risk is. This is because positivity rate is affected by who being tested - e.g. if only people who are showing symptoms or are very sick are tested then the positivity rate will be high but it does not tell us much about how many people in the general population have the virus.
Incidence tell us about the number of new cases of the disease within the population diagnosed within a specific period. The incidence rate provides a more informative picture of the general population's risk of contracting the disease. Ghana's current population is estimated population at 31,008,357. As of 23rd May, the country has reported 4,734 cases in the month of May giving an incidence rate of 0.015% for the period*. The incidence is lower than the positivity rates because groups at higher risks are mainly those being tested i.e those with symptoms and those who have come into contact with confirmed cases.
Prevalence looks beyond new cases to all cases within the population which provides information on how many people are living with the disease. Ghana's current population is estimated population at 31,008,357. The confirmed covid-19 case count is 6,808 which gives a prevalence rate of 0.022%. The prevalence is always higher than the incidence because it counts all cases not just new cases.
Infectivity provides information on how infectious a disease is i.e. how many people does an infected person transmit the disease to. To estimate this rate, we would need information on how many contacts are traced per case and how many of those cases test positive.
Case fatality provides information on the severity of the disease and the risk of dying from the disease among those diagnosed. With 32 deaths out of 6,808 confirmed cases, the case fatality rate is 0.47%.
Mortality provide information on the risk of dying from the disease in the general population. With 32 deaths in an estimated population of 31,008,357, the mortality rate is 0.0001%. Case fatality rates are typically higher than mortality rates because the denominator is smaller - diagnosed cases versus the total population.
One caveat: The calculated rates above are based on available figures. Covid-19 statistics are heavily influenced by the number and characteristics of people being tested and so these numbers can change as testing becomes more widespread.
* Some of these cases may be samples from April being reported in May due to backlog in testing but for the purposes of this example we assume they are cases diagnosed in May.
Update: We have put together a guide on these rates in an infographic here.
First of all what is a rate? The
Cambridge dictionary definition states that it is "The speed at which
something happens or changes, or the amount or number of times it happens or
changes in a particular period"
Epidemiologists use rates to
describe the frequency with which an event occurs in a
defined population over a specified period of time. In other words, it is used
to describe how fast an event is occurring in the population, in this case, how
fast covid-19 is occurring in the population.
Positivity Rates
Positivity tells us the proportion of those tested who are actually confirmed to have been infected with the disease. As of 23rd May, Ghana Health Service reports that they have tested 197,194 people out of which 6,808 have tested positive leading to a positivity rate of 3.44%. The positivity rate can tell us a lot or nothing at all about what the disease risk is. This is because positivity rate is affected by who being tested - e.g. if only people who are showing symptoms or are very sick are tested then the positivity rate will be high but it does not tell us much about how many people in the general population have the virus.
A more informative approach for that
would be looking at incidence and/or prevalence. This is especially relevant
for covid-19 due to the proportion of cases that are asymptomatic.
Incidence Rates
Incidence tell us about the number of new cases of the disease within the population diagnosed within a specific period. The incidence rate provides a more informative picture of the general population's risk of contracting the disease. Ghana's current population is estimated population at 31,008,357. As of 23rd May, the country has reported 4,734 cases in the month of May giving an incidence rate of 0.015% for the period*. The incidence is lower than the positivity rates because groups at higher risks are mainly those being tested i.e those with symptoms and those who have come into contact with confirmed cases.
Prevalence Rates
Prevalence looks beyond new cases to all cases within the population which provides information on how many people are living with the disease. Ghana's current population is estimated population at 31,008,357. The confirmed covid-19 case count is 6,808 which gives a prevalence rate of 0.022%. The prevalence is always higher than the incidence because it counts all cases not just new cases.
Infectivity Rates
Infectivity provides information on how infectious a disease is i.e. how many people does an infected person transmit the disease to. To estimate this rate, we would need information on how many contacts are traced per case and how many of those cases test positive.
Case Fatality Rates
Case fatality provides information on the severity of the disease and the risk of dying from the disease among those diagnosed. With 32 deaths out of 6,808 confirmed cases, the case fatality rate is 0.47%.
Mortality Rates
Mortality provide information on the risk of dying from the disease in the general population. With 32 deaths in an estimated population of 31,008,357, the mortality rate is 0.0001%. Case fatality rates are typically higher than mortality rates because the denominator is smaller - diagnosed cases versus the total population.
Notes
The examples also use national figures to
calculate rates but the statistics can be dis-aggregated for more detail:
Rates can be calculated for population
sub-groups who are at greater risk. 61% of confirmed cases are males
which would mean an estimated 4,153 males and 2,655 females have covid-19.
Estimated population numbers are 15,717,057 males and 15,291,339 females. That
would give a prevalence of 0.026% for males and 0.017% for females.
Rates can also be calculated by
geographic areas. The estimated population of Greater Accra
is 4,943,075. With 4,846 confirmed cases the prevalence rate is 0.10%, a
rate double the national prevalence of 0.02%.
* Some of these cases may be samples from April being reported in May due to backlog in testing but for the purposes of this example we assume they are cases diagnosed in May.