An international authority on epidemiology says that to completely stop Covid-19 transmission in a large population location, a 100 per cent vaccination rate is needed within that group.
Imperial College London infectious diseases expert Sir Roy Anderson is the co-author of the seminal Infectious Diseases of Humans: Dynamics and Control, one of the leading and most-cited texts within his field.
He told Breakfast achieving a 100 per cent vaccination rate is impossible because some people aren’t able to get the Covid-19 vaccine because of pre-existing medical conditions or because of hesitancy.
Because of this, Sir Roy said New Zealand needed to do two things: persuade as many people to get vaccinated and “move to the notion that you’re not preventing infection, but serious disease and mortality” in doing so.
He said he had arrived at the conclusions because real-world evidence of Covid-19 vaccines showed they were very effective at stopping serious illness.
“So, the question that many governments are asking is what fraction of the population do you have to immunise?”
With the Delta variant now dominant around the world, Sir Roy said each case would go on to infect an average of five to six other people.
“You take that figure, then you take the efficacy of the current vaccines,” he said.
He said effectiveness meant looking at how well a vaccine worked in the real world. This meant assessing its ability to stop Covid-19 infection and prevent people from becoming infectious, stop serious illness that would require hospitalisation, and stop death.
While the Pfizer vaccine is 95 per cent effective in clinical trials, this figure was lower in real life, Sir Roy said.
In a “quite small” fraction of vaccinated people, breakthrough infections were occurring and they can still pass on the virus to others, Sir Roy said. But, in the instance they contract the virus, their symptoms are milder.
“In other words, to stop transmission totally in a large population, you’d have to have everyone immunised,” Sir Roy explained.
Therefore, “it’s the third [criteria, of stopping death] we will become obsessed with” assessing, he said.
He said the consequences of that meant governments around the world, including New Zealand, would have to “move to this state” of battling the virus with vaccinations with the view of minimising harm, mortality, and preventing the health system from collapsing.
High vaccination rates would be key to this, he said.
Sir Roy said Covid-19 “is going to become endemic” for the “foreseeable future”.
He said this could mean countries would deal with it in a similar way to the flu, where vaccines are tweaked every year depending on the strains of Covid-19 that were circulating.
In a world before vaccination, pathogens acted as one of the most significant selective pressures on humans, he said.
“Prior to modern medicine, people died. Natural selection selected those who were more resistant and their children.”
Sir Roy said after about the 1920s, vaccines and two other medical interventions “probably saved more lives than any other aspect of medicine”.