Advertisement
Advertisement
Coronavirus pandemic
Get more with myNEWS
A personalised news feed of stories that matter to you
Learn more
Estimates for reaching herd immunity against emerging variants have been pushed higher in comparison to the original strain.

Coronavirus: no certainty on herd immunity until we know more about vaccines and variants

  • Early estimates for reaching herd immunity were based on the original strain but estimates for emerging variants are higher
  • As countries roll out different vaccines at different speeds along with control measures, there may be patchwork of risks in future

This is the sixth in a series about China’s plans to reopen its borders amid the Covid-19 pandemic. Here, Simone McCarthy and Zhuang Pinghui look into the problems in pinning down when “herd immunity” can be achieved.

Herd immunity – when enough of a population is immune to the coronavirus that its spread is limited – has been a goal of governments since the start of the Covid-19 pandemic.

Now, as a handful of nations have vaccinated at least half of their population, attention has turned to questions of when herd immunity can be achieved and what it will look like.

Crossing that unknown threshold could mean leaders of “zero-tolerance” countries open their borders, and offices and schools fully reopen in others.

But as new variants threaten to undercut vaccine efficacy and with vast portions of the global population that will not be vaccinated this year, experts say it remains to be seen when or whether this point can be reached globally and how it alters the risk posed by the virus that causes Covid-19.

“Many policymakers link herd immunity to the successful control or end of the pandemic, but it depends on how you define the end of the pandemic,” said Kwok Kin-on, an assistant professor at the Chinese University of Hong Kong’s JC School of Public Health and Primary Care.

“If we say zero infections is our ultimate goal, I’m not sure this will be achievable … there are lots of factors affecting herd immunity,” he said.

Two of those factors are how effective vaccines are and how easily the virus can spread. Both can change with the emergence of new variants of the virus.

What is driving China towards its coronavirus vaccination targets?

Early estimates for reaching herd immunity were based on the original strain of the virus. Back then, one infected person was estimated to spread the disease to between two and three others, according to Jodie McVernon, director of epidemiology at the Peter Doherty Institute for Infection and Immunity in Melbourne. But recent estimates for emerging variants have been higher, pushing the projected coverage needed for herd immunity up with it.

“We are really heading up to very high levels of coverage to suppress transmission completely,” she said.

That line was not yet clear, World Health Organization health emergencies chief Mike Ryan said this month, but “certainly north of 80 per cent coverage” was needed to significantly reduce the risk of an imported case causing an outbreak in countries that had had a low level of infection.

Early clues

Vaccine roll-out around the world has given some clues to how “herd immunity” could look and where pitfalls may lie.

Research published this month in the journal Nature Medicine showed that in Israel higher levels of Covid-19 vaccination with a product by Pfizer/BioNTech were linked to “substantial decline” in infection among unvaccinated children, indicating a community-level protective effect.
Brazilian researchers last month said their research in a small city outside Sao Paulo also showed those who were not vaccinated were protected when around three-quarters of the population – and 95 per cent of adults – were fully immunised with Sinovac Biotech jab.

Overall, deaths dropped 95 per cent and symptomatic cases fell 80 per cent, according to the study, described only in press releases.

But the research and local government data also indicated that despite the high level of vaccination, there remained cases in the city, underlining that high levels of vaccine coverage did not eliminate infection.

One reason is that vaccines are not perfect. Among vaccines approved for use by the WHO, clinical trial efficacy rates range from around 95 per cent effective for the Pfizer/BioNTech and Moderna vaccines, down to 51 per cent effective for Sinovac’s jab.

Experts said this meant some of a vaccinated population were also susceptible to symptomatic Covid-19, in addition to the unvaccinated population, at least before high levels of dose coverage were reached. It was hoped that cases of infection would be less severe in vaccinated people.

“With some of the vaccines the efficacy is around 80 per cent, which still means that one in five people who get exposed will show signs of infection,” said immunologist Joanna Kirman, an associate professor at New Zealand’s University of Otago.

Too early to tell if Covid-19 booster shots are needed: WHO scientist

Meanwhile, as countries ramped up vaccination levels there was a “delicate tipping point” to reach herd immunity, she said.

“Even when you’ve got half of the people vaccinated, you can still get the infection moving quite well through the community if it’s a relatively transmissible infection.”

That is the concern in countries such as the United States, where infection rates have fallen considerably alongside rising vaccination rates – now at over 45 per cent fully vaccinated – but it is still possible that the spread of a more transmissible variant or the relaxation of disease control measures could spark significant outbreaks among the unvaccinated.

It may also partly explain a recent spike in cases in Chile, even after 40 per cent of the population was fully vaccinated, mainly with a vaccine by Sinovac.

Sinovac vaccines create ‘immunity barrier’ in Brazilian town: study

However, CUHK’s Kwok said it was also necessary to think about differing levels of vaccine effectiveness because doses with lower efficacy could require more vaccine coverage to make an impact on the spread of the disease.

“One of the things is the new variants, which may affect the vaccine effectiveness, another is whether the adopted vaccine has high vaccine effectiveness or not,” he said.

Good, not perfect

Experts stress that vaccination is not all-or-nothing as vaccination levels gradually increase around the world and save lives.

“We won’t even eliminate it, not for a long period of time in most countries, but what we’ll do is to be able to get it to transmit at very much lower levels,” said Peter Collignon, professor of microbiology at the Australian National University.

In particular, Collignon said, the consequences for a population would be much less severe if the “vast majority” of the most at-risk population of older adults were vaccinated.

Meanwhile, and as countries roll out different vaccines at different speeds alongside various control measures, there is likely to be a patchwork of different results and risks in the next couple of years. Some experts said there could also be seasonal Covid-19 outbreaks much like the seasonal influenza.

Peter Chin-Hong, professor of medicine at the University of California, San Francisco, said countries could eventually safely reopen, especially if vaccines of higher efficacy were used and a high enough proportion of the population was vaccinated.

“But it doesn’t mean that countries that are using lower efficacy vaccines are in the lurch either,” he said.

“If we have confirmation that fully vaccinated individuals [infected in outbreaks] are spared from serious disease, hospitalisations and deaths – after all these are the outcomes we care about the most – that will inform our strategies in countries with a range of vaccines with differing efficacies,” he said.

Kirman of the University of Otago agreed, noting this could make a difference in terms of countries with strict borders and “zero tolerance” and accepting some disease spread: “We can put up with disease transmission of infectious agents that don’t land people in the ICU to any great extent.”

Global race

Now, the greatest barriers to reaching global herd immunity are supply constraints and uneven distribution, experts say.

More than 2.6 billion Covid-19 doses have been administered globally, but more than 1 billion of those have been given just in China, while a further quarter have been given in the wealthiest 27 economies, according to data from Bloomberg.

“Because the roll-out has been so painfully slow and transmission is still ongoing, variants of the future threaten to upend all of our progress,” Chin-Hong said.

Certain variants appear able to reduce protection offered by some vaccines, but according to available information the vaccines largely remain effective. It is not yet known if booster shots will be needed against new variants, but several are already in development.

“What I am most afraid of is the future creation of even more ominous variants, as we are far from breaking the chain of transmission globally,” Chin-Hong said.

Collignon said this might also mean some travel restrictions remained in place over the next couple of years as countries that had a high vaccination rate would want to keep out worrisome new variants.

“The more we get vaccinated [around the world], the less restrictions we need. But overall we’re still going to have a problem until the vast majority of the world is vaccinated.”

22