An international study has found mass vaccination is likely the only option for herd immunity against COVID-19.
Experts at Australia's Bond University looked at previous studies tracking the number of people with COVID-19 antibodies in their blood, and found far more people appear to have been exposed to the virus than official sources suggest - yet the deaths continue.
In at least half the countries looked at, at least 10 times as many people have been infected than the reported figures suggest. In one part of Iran, the official infection count was more than 700 times below the true figure, the study said.
The authors believe this means herd immunity is not possible without medical intervention.
"In all studies the estimated seroprevalences falls well short of that required for herd immunity suggesting that herd immunity is unlikely to be achieved without mass vaccination," the study, published in journal PLOS One on Saturday, reads.
"Additionally, infection fatality rates are shown to increase severalfold as the age of the people advance, further proving that herd immunity should not be pursued through the natural course of a pandemic."
The risk posed by the original COVID-19 goes up with age. There are signs the P.1 variant currently wreaking havoc in Brazil is more dangerous for youth than the original strain.
After a lull in February and March, infections and deaths are on the rise again - driven by a new wave of the UK variant hitting Europe and the P1 strain in South America.
The researchers also said antibodies against the original SARS-CoV virus, which killed hundreds of people in the early 2000s, declined in number after about six months - and the same might be true for antibodies generated by natural infections of SARS-CoV-2, which causes COVID-19.
The vaccine being used in New Zealand's rollout, made by Pfizer and BioNTech, has had great success in preventing both disease and transmission, both trial and real-world data shows. Six months after being inoculated, trial participants still have good protection against the virus, the latest data shows.
Some experts have said boosters might be needed further down the line, especially if the virus mutates to avoid prior vaccines.