Recent opinion polls have shown that fewer and fewer people are willing to get inoculated for COVID-19, raising the fear that a return to normal might prove elusive even after a vaccine is approved.
But some experts suggest that a low uptake of the vaccine might not delay life getting back to normal.
Preliminary analyses of European countries suggest that 20% vaccination, focused on healthcare workers and high-risk groups, may suppress transmission in the short term, according to Gabriela Gomes, a statistician at the University of Strathclyde in Glasgow, Scotland.
“I think most populations can be protected with less than 50% vaccination, at least in a relatively short term, one-two years, until acquired immunity wanes,” she said.
That is good news in a country that seems increasingly vaccine averse. A Gallup poll from July found that nearly two-thirds of Americans were willing to take a coronavirus vaccine. By late September, that had fallen to 50%.
Many epidemiologists suggest that “herd immunity” will be achieved when 60% to 70% of the population has been vaccinated or infected. Herd immunity occurs when a large portion of the population, the “herd,” becomes immune to a virus through a combination of people getting infected with the virus and people getting vaccinated. Once herd immunity has been achieved, it is harder for the virus to spread to the smaller part of the population that has not been infected or vaccinated.
Dr. Manoj Jain, an infectious disease physician at the Rollins School of Public Health, argues that a 50% vaccination rate may be sufficient to reach the 60% to 70% needed for herd immunity.
“There are people who have already been infected with the virus, and that could be 10% of the population,” Jain said. “Then there are people who may have had the virus. They’re not showing antibodies, but they have had a T-cell response. But we don’t know what percentage that is … but it could be 10% as well.”
At first glance, Jain’s numbers might appear overstated. About 10.6 million people in the United States have tested positive for COVID-19, about 3% of the population. But back in June, when barely 1% of the population had tested positive, Centers for Disease Control and Prevention Director Robert Redfield estimated that the total number of people infected might actually be between 5% and 8%. Furthermore, many studies have found T-cell responses in people who do not have antibodies. However, those studies had small sample sizes, and so how widespread the T-cell response is in the population is not yet known.
Another factor that will play a role in how long it takes to achieve herd immunity is the reproductive rate of the coronavirus, the number of people that one infected person will pass the virus on to, on average.
“There is a direct relationship between how infectious the virus is, which is what the reproductive rate measures, and what percentage of the population you need to be immune to effectively stop the virus,” said Hassan Vally, an epidemiologist at La Trobe University in Australia.
According to a formula used by epidemiologists, the percentage of the population that needs to be inoculated to achieve herd immunity is determined by the effectiveness of the vaccine and the reproductive rate of the virus. So, if a vaccine is 90% effective, as Pfizer says its vaccine is, and the reproductive rate of the coronavirus is, as most estimates put it, between 1.5 and 2.5, then between 37% and 67% of the population would need to be vaccinated.
And even if the U.S. doesn’t achieve full herd immunity, it might be able to scale back on preventive measures.
“If 50% become immune, then we’d need fewer preventive measures, but we wouldn’t be all the way back to the old ‘normal,'” said Tom Britton, a dean of mathematics and physics at Stockholm University in Sweden. “Perhaps it would be enough to have people stay home when having any symptoms and to wash hands more frequently.”
But getting to normal is still a long process even if enough people are inoculated, warned Vally.
“You don’t get back to normal immediately — the vaccine is no silver bullet,” said Vally. “You’ve got to roll out the vaccine, and that could take six to nine months. The logistics are quite difficult to navigate.”
Jain also suggested that we are headed toward a “new normal” in which we coexist with the coronavirus.
“Once we get to herd immunity … we can get back to as normal as possible, where the normal will include sporadic outbreaks of the virus which can be contained and managed, and we won’t have community transmission,” said Jain.
