Before the emergence of more contagious coronavirus variants, estimates placed the threshold for herd immunity at between 60 to 70 percent of the population. Yet, the growing dominance of the Delta strain, which is also more adept at dodging vaccines, has challenged this calculation based on high vaccination rates.
With the onslaught of the more contagious Delta strain, the State Serum Institute, Denmark's infectious diseases agency, has said it no longer believes it will be possible to achieve herd immunity through vaccination, implying that COVID-19 could circulate for years to come.
"It is not realistic to achieve herd immunity, understood as meaning that we will not see any spread of infection at all", SSI's acting academic director Tyra Grove Krause told the newspaper BT.
Herd immunity means that enough people are immune to infection from an illness that its reproduction number or R-rate (the number of people each infected person infects) falls below one, without any other anti-infection measures in place.
Before the emergence of more contagious variants such as the Alpha and Delta strains, estimates placed the threshold for herd immunity at between 60 to 70 percent of the population. However, the growing dominance of the Delta strain, which is also more apt at infecting people who are vaccinated, has challenged this calculus.
"If the vaccines were 100 percent effective against the variants that are in play now, and we had 100 percent vaccination coverage among those who are 12 years and older, then we could talk about achieving actual herd immunity against the Delta variant", Krause said. "But unfortunately this is not the reality, we cannot achieve that".
Krause admitted further COVID-19 outbreaks, but ventured that the new waves will look different from those witnessed in the past.
"We will still see infections with COVID-19, but we will not see serious complications to the extent that we have seen in the past, because the vaccine will protect us so well against serious disease", she said, assuring that the new waves per se will not be a cause for alarm. "We are going into a period where we will be able to tolerate much higher infection rates than we did previously because those who get infected will not become seriously ill. Our threshold for when an infection rate counts as 'high' has been shifted upwards".
Krause mused that the Delta strain also does not seriously threaten the vaccination programme, because there is "still a high level of protection against the disease left". According to her, this is in line with the purpose of the vaccines: "to prevent serious disease, not to eliminate viruses". Per Krause, the big question is how long the vaccines will remain effective.
Earlier, Iceland's chief epidemiologist Thórólfur Gudnason reached the same conclusion that vaccination hasn't led to the herd immunity experts had hoped for. In the past several weeks, the Delta variant has exceeded all others in Iceland, one of the most vaccinated nations in Europe, breaking previous infection records and making it clear that vaccinated people may easily contract and spread it as well.
According to Thórólfur Gudnason, half of those hospitalised have been vaccinated. He warned that the COVID-19 pandemic is not close to being over and won't be over until it's over everywhere, calling for a readiness to deal with upcoming challenges. To address the growing issue, Iceland has already reintroduced social restrictions, after abolishing them earlier this summer and being hailed by the media as the poster child that beat the pandemic.
Thórólfur Gudnason mused that there are only two ways to develop herd immunity: the spread of the infection itself and vaccination. "The question is whether we want to test herd immunity by opening and relaxing", he mused in an interview with the newsapaper Visir.
Iceland, a nation of 330,000 has seen over 8,700 cases and 30 deaths. Denmark, a nation of 5.8 million, has seen 323,000 cases and 2,500 deaths.