Herd Immunity Threshold for COVID-19 Could Be Just 43 Percent

The percentage of people in a population who need to be immune to the coronavirus to protect others, or the threshold of herd immunity, may be lower than previously thought, according to a study.

Herd immunity could potentially be achieved if 43 percent of a population is immune to the coronavirus, the authors of a paper published in the journal Science said. This is "substantially less" than the level of 60 percent given by vaccinating the population, the mathematicians from the University of Nottingham and University of Stockholm wrote.

However, the team said their estimate should be viewed as an "illustration" of how differences in a population affect herd immunity, "rather than an exact value or even a best estimate."

The study comes as questions remain about whether those who have caught the coronavirus that causes COVID-19 are immune to reinfection—and if so, for how long.

The authors defined herd immunity as the spread of disease declining and stopping, even after all preventative measures have been relaxed, because enough people could fight off. If such measures are eased before a certain level of immunity is achieved, the population in their hypothetical scenarios may be hit by a second wave, they said.

Generally, herd immunity against infectious diseases is achieved at between 50 to 70 percent, according to the authors, but this may not be possible for the coronavirus "without unacceptably high case fatality rates."

The researchers created a model to calculate how to achieve herd immunity against the coronavirus by taking into account factors that may affect the spread of the coronavirus, including the age of the individuals, which they divided into six categories. They also accounted for "normal," "low" and "high" activity levels, with low and high being half or double what they deemed normal, respectively. The model was based on the timeframe of the infection arriving in a hypothetical population on February 15, preventative measures starting on March 15, and being relaxed by June 30. Each infected person would spread the virus to 2.5 people in their scenarios.

The team acknowledged "more complex and realistic models" in the future may take into account other differences in populations, such as the virus spreading within households, at school or work, which are different across countries or regions. It is reasonable to assume, the team said, that these may bring the herd immunity level down even further, as more people will be infected in high contact environments.

The model works on the basis that those who catch and survive the coronavirus will have immunity for an extended period of time. Future models need to factor for the possibility that immunity may be lost quickly, and changes in populations—such as births and people aging and losing immunity—could affect it.

Co-author Frank Ball, professor of applied probability at the University of Nottingham, U.K., said in a written statement: "The more socially active individuals are then the more likely they are to get infected than less socially active ones, and they are also more likely to infect people if they become infected. Consequently, the herd immunity level is lower when immunity is caused by disease spreading than when immunity comes from vaccination.

"Our findings have potential consequences for the current COVID-19 pandemic and the release of lockdown and suggests that individual variation (e.g. in activity level) is an important feature to include in models that guide policy."

Holden Thorp, editor-in-chief of Science, wrote in a blog post that he and fellow editors discussed whether it was in the public interest to publish the findings, as they were concerned those wanting to downplay the severity of the pandemic and the need for social distancing would "seize on the results." But they decided it was important for the scientific community to have access to the model.

"Even if the model's most optimistic prediction of 43 percent as a herd immunity threshold is correct, none of the seroprevalence studies that we are aware of suggest that any country is close to achieving herd immunity. Continuing non-pharmaceutical interventions around the world is still of great importance," he said.

Daniel Davis, professor of immunology at the University of Manchester, U.K., who did not work on the study, told Newsweek via email: "The research is important and provocative but should not directly influence public advice or policy, because there are too many unknowns.

"Crucially, we still don't really know the extent to which infection with SARS-CoV-2 leads to protection against a second infection. Mathematical simulations are vitally important, but they only take us so far: we sorely need better understanding of the human immune response to SARS-CoV-2."

This article has been updated with comment from Daniel Davis.