The goal of the vaccination campaign against COVID-19 is herd immunity – the point at which so few people are susceptible to infection that the virus has nowhere to go.
In the early days of the pandemic, epidemiologists estimated that this would require the inoculation of about two-thirds of the population of the United States.
Now, many of those same experts say that number is almost certainly very low.
“If you really want herd immunity, where you get protection coverage over the country … you want about 75 to 85% of the country to be vaccinated,” said Dr. Anthony Fauci, the main disease worker infectious diseases last week. “I would say even closer to 85%.”
The change reflects a deeper understanding of how the virus spreads – that it bounces from one person to another more easily than previously thought.
The question of how many people should be vaccinated is of crucial importance, as the world embarks on the largest vaccination campaign in decades.
The goal of vaccination is not only to protect the individual who receives it, but also to place a fire blanket over a large enough portion of the population so that the fire will run out of fuel.
If few people are vaccinated, the virus will continue to find enough new hosts to spread – and will continue to stress the health system, delay economic recovery, need social distance and potentially increase again if vaccines become less effective over time.
Whatever the threshold for herd immunity, public health officials face a substantial challenge.
An early December survey by the Associated Press-NORC Public Affairs Research Center found that 46% of American adults planned to be vaccinated, while 26% would decline and 27% were still undecided.
A group of researchers found that anti-vaccination messages on social networks triplicate since the beginning of the pandemic.
A particular obstacle could be the vaccination of children and adolescents, a group that has not been hit particularly hard by the pandemic and for which vaccines are still being tested. But for 22% of the US population, they are important to any effort to achieve herd immunity and return to normal life.
When epidemiologists tried to model how many people would need to be vaccinated to bring the coronavirus to extinction, they compared trends in early transmission with those of other recent flu pandemics.
They looked at how the coronavirus had a longer incubation period, more asymptomatic spread and greater contagion – estimating that the pandemic would likely drag on for 18 to 24 months.
“It will probably not be stopped until 60% to 70% of the population is immune,” said a report published by infectious disease experts in April.
There are two paths to immunity: becoming infected with the virus and recovering or being vaccinated. Nor is it a guarantee.
Based on data from clinical trials showing that the effectiveness of the two authorized vaccines – Pfizer and Moderna – is excellent, but still imperfect, the limit for herd immunity rises to around 74%.
But experts say that even this calculation is still very simple.
“These numbers are useful for mental experiments, but they do not represent what is likely to be the way we control the virus or its impacts,” said Harvard epidemiologist Marc Lipsitch. “Offering a kind of magic number requires some very strong assumptions about these vaccines.”
Many factors can come into play. If the virus becomes even more transmissible, the limit for collective immunity will increase.
Targets may vary by location. In sparsely populated places where people adhere to social distancing guidelines, fewer people would have to be vaccinated to burn the virus.
“It will be the kind of thing that we will study for a long time,” said William Hanage, an epidemiologist at the Center for Communicable Disease Dynamics at Harvard.
Then there are the vaccines themselves.
They were authorized based on rapid clinical tests that showed that recipients were highly unlikely to develop symptoms of COVID-19 – but did not determine whether the vaccines actually prevent people from being infected with the virus or transmitting it.
The degree to which vaccines prevent transmission is very important in the equation for calculating herd immunity. In a bad case scenario, vaccines do so little to stop transmissions that collective immunity simply cannot be achieved by vaccination alone.
“At the moment, the jury has definitely not decided yet,” said Lipsitch. “If I had to guess, there will be a component of the herd’s immunity – I just don’t know how dramatic it will be.”
It may be that the extent of collective immunity depends not only on how many people have been vaccinated, but also on which people. The inoculation of the people most likely to spread it – people who live or work indoors, for example – can do much more to contain the pandemic than vaccinate people who live in relative confinement.
Considering all these unknowns, Fauci raised his estimate to 85% – and said it could be even higher.
The costs of not achieving herd immunity are substantial. If the virus continues to circulate widely, even some vaccinated people will develop COVID-19. Hospitals will continue to face virus outbreaks, depleting their resources and compromising their ability to treat heart attacks, strokes and other emergencies.
Meanwhile, the overall quality of life would continue to suffer. Schools, offices and restaurants would remain closed even to vaccinated people.
Experts say that until the virus is circulating at extraordinarily low levels – so that the risk of infection is close to zero – social detachment and wearing masks are here to stay.
The final answer to the question of how many people need to be vaccinated will not be known until collective immunity is actually achieved. When epidemiologists begin to see the test’s positive rate drop to extremely low numbers, they will know that the campaign is working.
But with the exception of smallpox, no virus that afflicts humans has been completely eliminated. Experts have been battling polio for decades, recently in conflict regions where vaccination campaigns have been halted.
They emphasize that, in the age of globalization, collective immunity must take into account almost every corner of the Earth – a pathogen anywhere remains a threat anywhere.
“I think it is extremely unlikely that we will be able to eradicate this virus,” said Hanage. “In reality, we have to accept that.”
“However, we must be able to get to the point where we will be able to live without it significantly damaging our lives, without leading to spikes that harm our health, or major excessive mortality – and that is what we are looking to achieve. “