With three safe and effective public coronavirus vaccines available in the United States, the conversation has shifted toward herd immunity, what it is, and how it plays a role in returning to normal.

So, what exactly is herd immunity, and what are the hurdles that Oregon—and the US—faces in achieving it?

What is herd immunity? 

Herd immunity is achieved when a population has sufficient indirect protection and has become immune to an infectious disease. This immunity, which is enough to prevent frequent spread of the infection, can be developed one of two ways: 1) vaccination or 2) previous infection.

For most highly contagious diseases, such as COVID-19, spread is exponential without any containment or immunity. In layperson’s terms: one person can infect two, two people can infect four, four people can infect 16. Achieving herd immunity means effectively slowing down a virus’s rate of infection to more of a linear slope (1 - 1). The more people that are immune, the fewer people the virus can latch onto.

How is herd immunity measured?

Herd immunity is typically recognized as a percentage. Most public health experts agree that in order to reach herd immunity, the percentage of the population that is immune needs to sit somewhere around 70 to 85. That threshold’s value, however, can change depending on a few factors: 1) the contagiousness of the disease, 2) the community’s ability to utilize safety and control measures, and 3) the natural evolution of infectious diseases.

Chunhuei Chi, a professor of international health at Oregon State University, says “there is no magic number,” and with the rise of concerning COVID-19 variants, such as the B.1.1.7, that threshold could grow higher.

“The threshold number, it’s not black and white. It’s in a range. But on average, that threshold number is proportionally higher if the virus or the bacteria are more contagious,” Chi says.

So how’s Oregon doing?

In Oregon, with a population of more than 4 million, the state would need to immunize roughly 2.8 million people to reach that 70 percent threshold. Right now, around 20 percent (just under 900,000) of Oregonians are fully vaccinated.

“Even without reaching community immunity, we can still prevent severe disease, hospitalizations, and deaths from COVID through vaccinations. This will allow us to return to normal, while still acknowledging that the pressures from new virus variants may require us to continue to be vigilant in our efforts to control the spread of the virus,” says Jonathan Modie, lead communications officer for the Oregon Health Authority. “As we work to achieve this goal, OHA wants to ensure every Oregonian who wishes to be safely vaccinated has the opportunity to do so. The pace of vaccination of eligible Oregonians will continue to be driven by the availability of vaccine supply to the state.”

Do we need the vaccine in order to achieve herd immunity?

Not really, but not vaccinating the population comes at the risk of natural immunity, which means letting thousands, potentially millions, of people die. Lest you want to sound like Ozymandias, vaccinations are the safest route to achieving herd immunity.

On returning to some semblance of normalcy, Chi says herd immunity is arguably the most effective means. There are other means, he says, to help us get back to normal, such as strict isolation and quarantine measures and continual practice of safe hygiene practices—wearing masks, washing your hands, physical distancing, etc. 

What hurdles do we face in safely achieving herd immunity?

A coronavirus vaccine has not yet been developed for children and teens under 16, but clinical trials are currently underway. Pending FDA approval, some health experts say a vaccine for children could be available by August. In Oregon, children under the age of 16 make up almost 20 percent of the population. 

As Modie suggests, the state’s vaccination rate also has a lot to do with it. Right now, Oregon is averaging about 26,000 vaccination doses daily, according to a media briefing on March 26. But the speed at which vaccines are rolled out is largely dependent on a functioning public health infrastructure. The pandemic, Chi says, has revealed the need for a vast overhaul of the US public health infrastructure.

Chi says vaccine hesitancy is another factor, and he says that group of people can be divided into two categories: those who are worried about vaccine side effects and allergic reactions, and those who, through political division or lack of trust in science, are resistant to the vaccine altogether. The former, he says, are easier to convince of the vaccine’s safety. The latter group might require a different form of outreach.

“The low trust and low confidence in science by a segment of the population is partially contributing to how we got here, with the United States ending up being one of the countries that has the worst consequences of this pandemic. That will continue to be a hurdle for how we get out of here,” Chi says. “Just like we have climate change denial, we also have pandemic denial. For those pandemic deniers, no amount of science or evidence or data will convince them. Instead, the point of communication is we want to get back to normal life, we want to help our economy.”

Listen: In this episode Footnotes, Chunhuei Chi talks about vaccine hesitancy and herd immunity.