5 Summary

This week, we introduced you to some aspects of epidemiology, including the concepts of incidence and prevalence. Incidence is given as the rate of new disease cases over a defined period of time, whereas prevalence gives the total proportion of people affected at any one time.

The value of antibody testing for SARS-CoV2 was outlined. For COVID-19, infected people usually undergo seroconversion 7-14 days after infection, and they then remain seropositive for many months, possibly years. Infection with the virus induces both S-antibodies and N-antibodies. The presence of N-antibodies distinguishes previously infected individuals from those that have been vaccinated against the spike protein.

Seroprevalence, the proportion of people who are seropositive for antibodies, can be used to compare the cumulative level of infection in different regions or over a period of time. This information contributes to understanding disease spread and how it can be controlled. Particularly important is knowing what proportion of infected people are asymptomatic – in the case of COVID-19, up to 40% of cases were asymptomatic. Tests for infection (eg PCR) often underestimate the prevalence of the disease, when there are large numbers of asymptomatic cases, as they are less likely to come forward for testing. For this reason, random sampling in the community is the most reliable way of getting good estimates of disease prevalence.

Finally you used the virtual laboratory to identify sera with N-antibodies as evidence of previous COVID-19 infection. You will take this investigation one step further next week.

Now go to Week 6.