Data from Brazil is showing aiming for natural herd immunity from coronavirus is a 'very dangerous thing to try', according to Professor Luke O'Neill.
The Trinity College immunologist says many of the advocates of the approach have 'gone quiet' as more evidence indicates it's not an effective strategy.
Brazil has seen the third highest number of cases (nearly 8.5 million) and the second highest number of deaths (over 200,000) during the pandemic.
The city of Manaus in the Amazonas region saw huge case numbers during the first wave, but a recent surge of infection in the area has seen hospitals overwhelmed.
It's led to a fresh debate about herd immunity, which is the idea that a whole population will be protected against a virus once enough people have immunity to it.
While this can be achieved through vaccination, others have claimed it can be achieved 'naturally' as a result of many people becoming infected and recovering from the virus.
Professor O'Neill told The Pat Kenny Show that Manaus was a big focus due to the volume of cases reported during the spring and summer.
He explained: “Back in September, the health secretary said ‘we’ve had a huge amount of infection. We would now have herd immunity… don’t expect a second wave’.
“Disastrously, they’re in the middle of a huge upsurge in cases. In fact, it’s gotten so bad, the special cemetery they built… is now full.
"It’s just a shocking example of how you cannot get to herd immunity any way quickly or safely."
He suggested that "herd immunity advocates have got quiet" as a result of the data.
He pointed to the 'Great Barrington Declaration', a declaration signed last year by three international public health experts voicing concerns about lockdown policies.
Professor O'Neill said: “The bottom line is you can’t get to herd immunity naturally without causing a huge amount of illness and death."
He also noted that the policies and approach of President Jair Bolsonaro 'didn't help', as the Brazilian leader has frequently dismissed the danger of the virus.
Professor O'Neill said there's now a 'big spotlight' on Manaus as people look at the decisions made and advice given to citizens before the recent surge.
There was news last week that there may be a temporary delay to Pfizer's COVID-19 vaccine deliveries in Europe as the company upgrades their production methods.
Professor O'Neill said it's not something to worry about, as it means we'll see a 'massive' ramping up of production to at least two billion doses this year.
He explained Pfizer is still 'way out front' and has a clear advantage in being the first vaccine available to countries.
However, various other jabs are making progress through the testing and approval processes.
Professor O'Neill said Johnson & Johnson is running two huge trials - one testing a single shot dose, and another a double shot dose.
He said: “The single shot trial… they’ve just published data. They measured antibodies and t-cells in people… 90% of people had a high level of antibody by day 29, and 100% of people had it by day 57.
“T-cells… were really strongly boosted as well. That would suggest the single-shot would definitely work. They’re going to release the data, they said, on January 21st… we’ll see the first scientific report.
“We’re expecting that one to be approved by mid-February. The European Union has bought 400 million doses… some of us are saying that might be the one that might begin to dominate the market, certainly say three, six, nine months from now."
Meanwhile, a Chinese vaccine called Sinovac is also showing some promising results - although Professor O'Neill stressed there hasn't been much data released on it yet.
He explained: "There’s one or two early phase two type studies… but the big phase three hasn’t been published yet.
“Instead, we’re hearing from four different places of levels of efficacy - one says 91%, another says 50%. They reckon it will probably settle at around 70% efficacy… that’s really good.
“That one is more of an old-fashioned vaccine… the whole virus is in it. It’s been inactivated, so it can’t infect you.
"That would work against any variant… it could be very useful if these variants continue.”
Elsewhere, Professor O'Neill said the AstraZeneca / Oxford jab is 'bound to be approved' by the EU later this month - and that the Government is right to start planning to 'get it into fridges' as quickly as possible.