Skip to Main Content

As coronavirus infections decline in the U.S., it seems appropriate to celebrate the triumph of vaccines over viruses. But how much of the credit do vaccines deserve? Less than you might expect.

Don’t get me wrong: I believe in vaccines. I got vaccinated as soon as I was eligible and am 100% behind the goal of getting everyone on the planet vaccinated. Yet there are other factors also at work in quelling a pandemic.

Following patterns from previous pandemics, the precipitous decline in new cases of Covid-19 started well before a meaningful number of people had been vaccinated.


Nearly 50 years ago, medical sociologists John and Sonja McKinlay examined death rates from 10 serious diseases: tuberculosis, scarlet fever, influenzae, pneumonia, diphtheria, whooping cough, measles, smallpox, typhoid, and polio. In each case, the new therapy or vaccine credited with overcoming it was introduced well after the disease was in decline. More recently, historian Thomas McKeown noted that deaths from bronchitis, pneumonia, and influenza had begun rapidly falling 35 years before the introduction of new medicines that were credited with their conquest.

These historical analyses are relevant to the current pandemic.


New cases of Covid-19 peaked in early January 2021. Since then, cases retreated from more than 300,000 per day on Jan. 8 to around 55,000 on Feb. 21. Vaccines were first given emergency use authorization toward the end of December 2020. By Feb. 21, only 5.9% of Americans had received two shots, yet there had been an 82% decline in new cases.

Daily Covid Cases
New cases of Covid-19 peaked on Jan. 8, 2021. By Feb. 21, only 5.9% of the population had been fully vaccinated, but cases had fallen 82%. J. Emory Parker/STAT

To be sure, coronavirus vaccines are a remarkable accomplishment. But even a vaccine that’s 95% effective can’t take full credit if it is introduced on the back of a naturally receding epidemic. Timing is everything.

There are two ways to develop immunity: natural infection and vaccination. The best explanation for declining rates of Covid-19 appears to be previous infections, which vary considerably from state to state.

Individuals with confirmed Covid-19 diagnoses are only the tip of the iceberg. Although estimates vary, the most recent study from the National Institutes of Health suggests that about five people were infected with SARS-CoV-2, the virus that causes Covid-19, for each person with a confirmed case. Multiplying known cases by five yields a rough estimate of the number of people who may have been infected. I performed a simple calculation of what I call the natural immunity rate by dividing my estimate of the number of people naturally infected by SARS-CoV-2 by the population of the state.

By mid-February 2021, an estimated 150 million people in the U.S. (30 million times five) may have had been infected with SARS-CoV-2. By April, I estimated the natural immunity rate to be above 55% in 10 states: Arizona, Iowa, Nebraska, North Dakota, Oklahoma, Rhode Island, South Dakota, Tennessee, Utah, and Wisconsin. At the other end of the continuum, I estimated the natural immunity rate to be below 35% in the District of Columbia, Hawaii, Maine, Maryland, New Hampshire, Oregon, Puerto Rico, Vermont, Virginia, and Washington.

The table below summarizes how these two groups of states with high and low natural infection rates differ. By the end of 2020, new infections were already rapidly declining in nearly all of the 10 states where the majority may have had natural immunity, well before more than a minuscule percentage of Americans were fully vaccinated. In 80% of these states, the day when new cases were at their peak occurred before vaccines were available.

In contrast, the 10 states with lower rates of previous infections were much more likely to experience new upticks in Covid-19 cases in March and April. The date with the highest number of new cases occurred before vaccines were available in only 30% of these states. By the end of May, states with fewer new infections had significantly lower vaccination rates than states with more new infections.

In other words, states with low natural immunity, like Oregon, Vermont, and Washington, that had vaccinated more of their citizens were also experiencing more new Covid-19 cases. And despite lower vaccination rates, high natural immunity states like North Dakota, Tennessee, and Utah may have had fewer cases because more of their citizens were protected through natural immunity from previous infections.

Public policy may have also played an unexpected role. In the 2020 presidential election, Donald Trump carried seven of the 10 states with high natural immunity, while Joe Biden carried all 10 states with low natural immunity. Among the 10 states with high natural immunity, 90% did not have active mask policies, while 90% of states with low natural immunity had mask policies in place through May.

Variable 10 States with Highest Natural Immunity Rates 10 States with Lowest Natural Immunity Rates
Estimated previous infections (confirmed cases x 5) 61% 26%
Fully vaccinated 39% 45%
14-day change in cases -9% +9%
States won by Trump 70% 0%
States with mask policy in effect 10% 90%
States with peak infections before vaccines available (Jan. 1, 2021) 80% 30%

Data from The New York Times, values through May 2021

So, what happened? First, states that enacted mask and related policies had fewer new cases of Covid-19. That is certainly good news. But there may have been a downside: Mask and other mandates that successfully lowered infections may have left unvaccinated people vulnerable to SARS-CoV-2 because fewer had acquired natural immunity. Conversely, states without mask policies suffered higher rates of infection that resulted in more natural immunity, offering more protection before vaccines were available.

Although vaccines and previous infections are alternative pathways to immunity, vaccines are by far the safer route to it. The cost for some states to achieve high natural immunity was the heartbreak of intolerable rates of illness, hospitalization, and death. Suggesting that people risk getting infected to achieve immunity is unacceptably irresponsible.

To be prepared for future pandemics, the U.S. — and other countries — need to learn what really happened across the course of this one. The answer is not likely to be simple, since there is plenty of nuance overlaid on ever-changing trends.

For example, in contrast to my analysis, the Washington Post reported on June 14 that coronavirus infections are falling where people are vaccinated and rising where they are not. The Post’s analysis is correct: There is a modest correlation between current state-level vaccination rates and the most recent reports of new cases. But it ignores longer-term trends.

States with low natural immunity, despite administering more vaccines, were more likely to have surges of Covid-19 in the spring. As the pandemic was receding, declines in low natural immunity states may have appeared larger because they were falling from higher spring peaks. Most states with high natural immunity had achieved low new case rates by December, leaving less room to observe declines. A set of graphs is available here.

Some analyses also concentrate on small geographic areas. Sweetwater County, Wyo., where only about 27% of residents are fully vaccinated, has received significant media attention. The county recently led Wyoming for increases in new cases. Wyoming, in turn, leads the U.S. in new infections. But Sweetwater County has only 42,343 residents. Minor — and perhaps transitory — changes in the number of cases can appear as a large percentage change.

In contrast to the suggested spike, data from the New York Times show that Sweetwater’s seven-day average for new cases has been relatively steady for the last few months and recently declined from a high on June 1. And though Wyoming has a very low statewide vaccination rate, new cases had dropped by mid-February — before many people could have been vaccinated — and have remained fairly steady through the spring.

Epidemiology is rarely simple and it will take some time to sort through these conflicting interpretations.

But one trend appears to be clear. As in previous pandemics, the rapid fall in new cases preceded the widespread distribution of vaccines. Although vaccines deserve much credit for declining rates of Covid-19, the protection provided by natural infection has been underappreciated. Emerging evidence shows that previously infected people have effective and durable immunity that rivals or exceeds the benefits of vaccines.

When historians look back at this pandemic, they might report that credit for its resolution should be shared by vaccines, natural immunity, and other public health measures. But even though vaccines did not initiate the decline in Covid-19 cases, they are the best tool available for assuring that the smoldering fire of it is extinguished.

Robert M. Kaplan is a faculty member at Stanford Medicine’s Clinical Excellence Research Center and a distinguished professor emeritus at the UCLA Fielding School of Public Health.

Create a display name to comment

This name will appear with your comment