3.7.12Additional lessons

Evaluating COVID-19 Data- Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 101

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Part 1 of 5Reading Epidemic Data

Check what each axis shows, including units, the time period and whether the vertical scale is linear or logarithmic.

Decide whether numbers are absolute counts or rates per 100 000, because only rates allow fair comparison between countries of different size.

Identify trends, such as the shape of waves, peaks and the time lag between cases, hospital admissions and deaths.

Consider how the data were collected, for example whether testing rates changed and so altered the number of reported cases.

Draw conclusions that the data support, and avoid claiming that correlation proves cause.

Case fatality rate

Deaths divided by confirmed cases, multiplied by 100, gives the percentage of identified cases that died.

Incidence

The number of new cases in a time period, often expressed per 100 000 people to allow comparison.

Vaccine efficacy

The percentage reduction in disease among vaccinated people compared with an unvaccinated placebo group in a trial.

Percentage change

Change divided by the original value times 100, useful for comparing cases before and after an intervention.

Measure	Formula	Meaning
Case fatality rate	Deaths ÷ cases × 100	Risk of death among cases
Vaccine efficacy	(Risk placebo − risk vaccine) ÷ risk placebo × 100	Reduction in risk
Incidence rate	Cases ÷ population × 100 000	Cases per 100 000

Evaluating Reliability

Sample size

Trials with tens of thousands of volunteers give narrow uncertainty, so their results are more trustworthy.

Controls

Comparing vaccinated people with a placebo group, ideally in a double-blind design, removes bias.

Data quality

Under-reporting, differences in testing and delays in recording deaths limit comparisons between places.

Confounding variables

Age, health, season and lockdown measures also change case numbers and can be mistaken for the effect of one factor.

Question

In a trial, 160 of 18 000 people given placebo developed COVID-19 and 8 of 18 000 given the vaccine did.
Calculate vaccine efficacy.
Another region reports 2 000 deaths among 80 000 confirmed cases;
- calculate the case fatality rate.

Risk in placebo group = 160 ÷ 18 000 = 0.00889; risk in vaccine group = 8 ÷ 18 000 = 0.00044.
Efficacy = (0.00889 − 0.00044) ÷ 0.00889 × 100 = 95%.
Case fatality rate = 2 000 ÷ 80 000 × 100 = 2.5%.

Answer Vaccine efficacy is 95%, and the case fatality rate is 2.5%.

Case fatality rate Percentage of confirmed cases that result in death.
Vaccine efficacy Percentage reduction in disease in vaccinated people compared with a placebo group.
Placebo An inactive treatment given to a control group.
Confounding variable A factor other than the one being tested that affects the result.

Comparing raw case numbers between countries of different population size.
Confusing case fatality rate (among cases) with the death rate of the whole population.
Concluding that a vaccine caused a fall in cases when other factors changed at the same time.

Last updated: 28 October 2026