According to the CDC, COVID-19 confirmed cases are those with positive laboratory tests results.
Looking only at raw case numbers won't tell how much of the population is infected.
This can be achieved by adjusting for that population
size by using one case per 100,000 people, allowing valid comparisons with other r
egions or countries with different population sizes.
Positivity rate:
The positivity rate indicates how hard or easy it is to find a case, reflecting
both the spread of COVID-19 and how widespread testing is. If the rate of
positive tests is 20%,
there will be no need to look hard to find a case versus 1%, which means
that many tests have to be done to get one positive one.
Rate of infection:
The estimation of infection rate in a population is based on the "R0,"
or reproduction number. R0 is the average number of people who will catch
the disease from a single infected person in a population that's never seen
the disease before. So, if R0 is 3,
one case will create an average of three new cases. When that transmission
rate of infection occurs at a specific time, it's called an "effective R," or "Rt."
When the R0 is less than 1, that means the epidemic is under Control;
A reproductive number higher than one implies a progressive
increase in cases, which translates into epidemics. Therefore,
the reduction in this value must guide preventive strategies towards
transmission control.
The rapid increase in clinical cases of COVID-19 suggests high transmissibility.
However, the basic reproductive number
estimates in the literature vary widely. Several studies
indicated that the coronavirus's most probable basic reproductive
number (R0) is between two and four.
How do you think the chain of transmission would be affected if some individuals in the population are not susceptible to the infection (for example they are already infected)?
Attack rate :
The term attack rate measures the number of cases in the population
relative to the total number of people at
risk of being infected. In epidemiology, it is used to define the
risk of contracting a disease during a specified time interval
i.e., outbreak duration).
- The overall attack rate refers to the total number of new cases divided by the total population:
Attack Rate (%) = No. of new cases of disease during
time interval/Population at risk at the start of time interval x 100
- The secondary attack rate can help document the difference between community
transmission and disease transmission in a closed population, for example, a household.
Secondary Attack Rate (%) = No. of new cases among
contacts/Total number of contacts at risk x 100
A case definition or set of criteria to define the disease of
interest must first be developed to calculate an attack rate. Case definitions may be
based on a constellation of clinical signs (e.g., fever, cough, vomiting, or diarrhea)
or laboratory tests (e.g., RT-PCR, Antigenic tests).
The number of people who meet the case definition is represented
in the numerator of the attack rate. The denominator of the attack rate is the number
of people who are at risk of becoming ill.
At-risk individuals are those persons who had the opportunity to be exposed to the disease—for
example, all individuals who ate a particular food item.
The "at risk population" :
The at-risk population is defined as those who are susceptible to developing
the disease carried by the attacking pathogen (and who have no immunity to it)
and are at a higher risk of becoming ill than others.
