With COVID now endemic, modelling suggests targeted protection will be more effective than blanket measures

• In New Zealand, the government removed all remaining public health measures last week.
• But although the emergency is over and the disease is rapidly becoming endemic, the risk of new variants remains.

The basic reproduction number

• Back in 2020, we heard a lot about the basic reproduction number or R0.
• This is the average number of people someone infects when the whole population is susceptible to the disease.
• With a susceptible population, if R0 is above 1 the disease spreads exponentially.
• This situation prompted governments around the world to implement intensive response measures, including lockdowns, to prevent health systems from becoming completely overwhelmed.

Controlling the disease

• Effective control measures should reduce the number of contacts infectious people have, or the risk of infection per contact.
• That’s certainly true, but how much effect do control measures realistically have for a virus like SARS-CoV-2?
• But the effective reproduction number – the average number of people someone infects at the present time – is much closer to 1.

The maths of immunity

• People may have acquired immunity through vaccination, but the protection vaccines provide against infection with current Omicron variants is relatively low and short-lived.
• The majority of immunity comes from previous infections, including infections in vaccinated people.
• (This doesn’t mean that getting infected to get immunity should ever be a goal, but it is an important side effect).
• If R0=10, the maths is even more dismal: the same control measure only gives a 3% reduction in infections.

Targeted protection

• The susceptible-immune binary is a simplification because immunity is not black and white but shades of grey.
• Interventions targeted towards vulnerable groups are likely to be more effective than blanket measures.
• Importantly, although reducing infection rates in the long term is difficult, vaccines provide direct protection for those who take them and continue to be highly effective at preventing severe disease.
• He is an invited expert member of the Communicable Disease Network of Australia and between January 2020 and May 2022 was an invited expert member of the Australian Health Protection Principal Committee.