Now that COVID-19 vaccinations are underway, what should countries do next in their battles against the pandemic? In Europe, very different paths are being discussed.

On Friday, the Financial Times reported that senior government scientific advisers in the U.K. are considering the idea of letting a “big wave of infection” sweep the nation once those over age 50, a relatively at-risk group, are vaccinated.

“It boils down to what we, as a society, are prepared to accept,” Mike Tildesley, a member of the government’s Scientific Pandemic Influenza Group on Modelling, told the newspaper. “It’s possible you could run hot in terms of cases and low in terms of number of hospitalizations and deaths.”

Such a strategy may make the lifting of the U.K.’s lockdown more straightforward. However, in some EU countries such as Ireland and Germany, the opposite approach is under consideration.

On Thursday, the World Health Organization warned Europe against adopting a “zero COVID” policy—aiming for the eradication of all cases—just yet.

“Elimination is something we want, in principle, for any disease, for any pathogen, and it can be a very powerful working incentive,” WHO regional director for Europe Hans Henri P. Kluge told reporters. “But whether we are at the stage now—to put targets for a ‘zero COVID’ strategy—is still a different ball game.”

Zero COVID

The zero-COVID approach has worked out well in Australia and New Zealand, which have almost completely suppressed the disease by banning incoming travel—something that is obviously more easily done in countries with no land borders to any other countries—and instituting very hard lockdown measures. (For an example of the latter, the Australian state of Victoria just went into lockdown, complete with travel restrictions, following the emergence of five cases alone.)

Germany is literally in a very different position, having land borders with nine other countries. So while leading virologist Christian Drosten last month called for continued lockdown measures by saying, “It would be absolutely desirable to at least aim for zero now,” Health Minister Jens Spahn argued at the time that an elimination policy would be unrealistic.

“I don’t see zero as a permanent target, [as something that] can work in a country like Germany with our situation,” Spahn said.

Opposition politicians in Ireland have also called for a zero-COVID policy, which the government has rejected, again because of the issue of land borders—in this case, the boundary between the Republic of Ireland and Northern Ireland, a province of the U.K.

“We simply couldn’t realistically seal the borders of this country and stop movement of people in and out,” said Tony Holohan, Ireland’s chief medical officer, a couple of weeks ago.

The WHO’s Kluge said Thursday that Europe should for now concentrate on aligning vaccine rollouts with the management of lockdown measures.

In the face of the more transmissible coronavirus variants that are now spreading, he said, “the biggest mistake is to lower our guard” prematurely.

‘Big wave of infection’

What the U.K. government advisers are talking about is essentially the calculated lowering of the nation’s guard, once those over age 50 are inoculated.

Younger people are more likely to contract COVID-19 without requiring hospitalization. However, there are also long-term considerations in play. Tildesley told the FT that even if just 10% of the population remains unvaccinated, millions of people could still be affected by so-called long COVID.

“Long COVID” is a term that refers to the lingering effects of the disease—including cognitive dysfunction, fatigue, and respiratory problems, among many others—even after the infection has passed, and even if the initial illness was only mildly symptomatic. Because of the novelty of the disease, no one knows how long these symptoms might persist.

The British government has promised to offer a vaccine to every adult in the country by September, which should make a huge difference in driving down infection rates. But that’s more than half a year away.

Allowing a “big wave of infection,” as one unidentified government adviser put it to the FT, is not a new idea. It would involve some of the same tactics that were under discussion in last year’s “herd immunity” debates.

“Herd immunity” refers to the threshold at which a country has enough immune people to protect the whole society. It can be achieved through mass vaccination or, some argue, naturally. WHO director-general Tedros Adhanom Ghebreyesus counterargued last October that trying to achieve COVID-19 herd immunity through exposure rather than protection would be not only ethically problematic, but possibly ineffective too; we don’t yet know how long immunity will last, and how strongly.

What is being discussed in the U.K. is not quite the same thing; the plan there is to achieve herd immunity through vaccination. But it would also mean accepting higher risk for a limited period until the threshold is reached, and some say the risk would be too high.

“Letting a big wave of infection flow through the U.K. would mean hundreds of thousands of people with long COVID, and further virus adaptation and spread that may threaten vaccine effectiveness,” tweeted Deepti Gurdasani, a clinical epidemiologist and machine learning lecturer at Queen Mary University of London, on Friday, in response to the FT article.

However, there is another aspect to this debate, namely vaccine inequality—the issue that is on track to see vast swaths of the world remain unprotected long after richer societies achieve herd immunity.

One member of the U.K. government’s advisory group told the FT that “if we start vaccinating the young, at that point we will be depriving other people.” This also appears to align with the views of the WHO chief.

“It’s right that all governments want to prioritize vaccinating their own health workers and older people first,” Tedros said in January. “But it’s not right that younger, healthier adults in rich countries are vaccinated before health workers and older people in poorer countries. There will be enough vaccine for everyone.”

Ultimately, a failure to properly vaccinate poorer countries could rebound on richer countries as the virus finds space to mutate. But the same result could materialize if COVID-19 is allowed to spread anywhere—meaning lockdowns may be the only safe option for a while yet.