Herd immunity, also known as “population immunity,” refers to the situation when enough members of a population, or “herd,” develop immunity to a pathogen to prevent further outbreaks. But not all infectious diseases can be controlled through herd immunity. Success depends on two factors:

• The percentage of the population that must develop immunity before the disease is controlled
• How long immunity lasts

The percentage of immune individuals required to control a disease depends on how contagious the pathogen is. To illustrate, let’s consider two of the most contagious viral diseases: measles and norovirus (the cause of the stomach flu). We measure contagiousness, or how easily a disease is transmitted, using a term known as the reproductive number. For example:

• Measles: For every new case, we can expect a reproductive number of 14 to 18 additional cases.
• Norovirus: For every new case, we expect a reproductive number of 10 to 14 cases.
• Influenza: By comparison, the reproductive number of influenza is less than 2.

The more easily a disease is transmitted, the more immunity it takes in the population to prevent outbreaks. So, for measles and norovirus, more than 95 percent of the population would need to become immune to prevent outbreaks.

Let’s take the example of measles and norovirus again. Natural immunity to measles can last a lifetime, so over time, it’s easy to see how a population could achieve 95 percent herd immunity. On the other hand, immunity to norovirus lasts only about six months. In order to achieve herd immunity to norovirus, 95 percent of the population would all have to get the stomach bug every six months! Clearly, herd immunity is not possible for norovirus and other viruses that don’t stimulate long-lasting immunity.

Long before COVID-19, scientists understood the transmission cycle and immunity of the four other coronaviruses that are very common causes of respiratory infections in humans. Like other respiratory viruses, such as influenza and RSV, immunity to coronaviruses is also very short-lived – up to nine months at the longest. This why these viruses are considered seasonal and we see spikes in cases every year because the “herd” never develops enough immunity.

Thanks to new research from renowned virologists and epidemiologists, we now know that natural immunity to the SARS-CoV-2 virus lasts only three to nine months. The reproductive number for COVID-19 is between 2-3. That means that in order to achieve herd immunity to COVID-19, 70 percent of the population would have be to infected every nine months! In other words, it’s impossible to achieve herd immunity to COVID-19 through widespread transmission.

We’ve found that measuring immunity to SARS-CoV-2 is difficult. Antibody tests are not accurate enough to prove whether someone is immune, and the level of immunity varies depending a number of factors including how severe the symptoms were. Based on multiple surveys of various groups around the country, it appears that, at most, only 10 percent of the population have had COVID-19, and many of those may not have developed full immunity.

In order for 70 percent of the U.S. population to be infected in a nine-month period, we’re talking about 230 million cases, or about 850,000 cases a day. Since we’re hitting more than 100,000 a day right now, it’s not inconceivable that that type of overwhelming transmission is possible. But the effects would be devastating. What would those nine months look like?

• More than five times more people would get sick than in a typical flu season
• More than 1 billion work days lost
• An estimated $109 billion in lost income
• More than 42,000 patients would be hospitalized every day
• More than 11 million hospital admissions
• More than 2.3 million Americans would die of COVID-19 (if the current U.S. case fatality ratio of 1 percent holds). That is more than die of heart disease, cancer, accidents, car accidents, stroke and pneumonia every year, combined.

These numbers clearly show that achieving herd immunity to COVID-19 through infection is not only impossible but would come at a terrible cost to healthcare systems, the economy and our lives. But there is hope.

The goal of vaccines is to stimulate long-lasting immunity without the collateral damage of natural infection. We don’t yet know how long lasting the immunity from the vaccine candidates will be, but if they prove to have a good safety profile, they will represent the only viable way of achieving herd immunity, even if it means yearly vaccination.

Until a vaccine is widely available, we must suppress transmission by limiting contact with non-household members, and wearing masks and distancing when contact with others is unavoidable. These measures work and will limit unnecessary deaths and economic impact while we wait for an effective vaccine.

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Brandon Webb, MD, is an infectious diseases specialist at Intermountain Medical Center.