Six months into the campaign to vaccinate the US population against Covid-19, stark differences in vaccination rates have emerged between the states with the highest and lowest rates. However, states that have been slower to vaccinate have not paid a high price in terms of new case outbreaks, thanks in part to what scientists refer to as the open-air effect.

Many of the states with the lowest proportions of people who have received at least one vaccine shot are in the Deep South, including Mississippi, Georgia, Alabama, and Arkansas, which avoided large outbreaks last spring only to see cases spike in the hot summer months. Most of these states also avoided a spike in cases early this year, despite the fact that many northeastern states, including Vermont, Massachusetts, and New Hampshire, which now have some of the highest vaccination rates in the country, saw cases rise during the winter and early spring.

According to epidemiologists and research, residents in southern states have largely faced a lower risk of transmission during the winter and spring months because they have been able to spend more time socializing in the open air, where the virus disperses more easily. And, unlike their northern counterparts, they have not had to rely on heating systems that dry out the indoor air. Several studies have found that the SARS-CoV-2 virus spreads less easily outside and in more humid environments.

However, doctors and public-health officials are concerned that as summer approaches, warm-weather states with lower vaccination rates may be vulnerable to a new round of Covid-19 outbreaks as the heat forces people to spend more time in cool, air-conditioned places. According to a Wall Street Journal analysis of Centers for Disease Control and Prevention data, 37.9 percent of people in the ten states with the lowest vaccination rates—seven of which are in the Deep South—have received at least one vaccine shot, compared to 50.5 percent of the total U.S. population. The average first-dose rate in the top ten most-vaccinated states is 62.6 percent.

From late February to late May, new confirmed cases in these Southern states trended consistently downward. In Mississippi, where only 34% of the population has received at least one vaccine dose, making it the least-vaccinated state in the country, the seven-day rolling average of new cases has dropped by about 72 percent since March 1 to 162 per day, bucking a consistent downward trend. Since March 1, average daily case counts in Georgia have decreased by about 83 percent, 81 percent in Arkansas, and 51 percent in Louisiana, capping a steady downward trend despite all three states having first-shot vaccination rates of less than 40 percent. Nine of the ten states with the highest vaccination rates in the country, which, with the exception of Hawaii and New Mexico, are all in the northeast, experienced spring surges or sustained upward trends in new infections during the same period before high vaccination rates began to knock them down.

Vermont, now the nation’s most-vaccinated state, with 70.2 percent of residents at least partially vaccinated, saw the seven-day moving average of cases nearly double to nearly 190 per day between March 1 and April 4, the highest level since 2021. It has since dropped precipitously to an average of 27.3 new cases per day. Cases in Maine, Massachusetts, and New Hampshire all increased in late March and early April before declining by about 32%, 83%, and 76%, respectively, compared to March 1 levels.

The disparity supports the notion that the amount of time people spend outside—and how little time they spend in low-humidity indoor environments breathing recirculated air that has been artificially heated or cooled—may play a significant role in how quickly the virus that causes Covid-19 spreads. Outdoor time and environmental factors such as humidity and air circulation are far from the only considerations. Vaccination rates are important, but mask mandates, restrictions on public gatherings, and geographic factors such as population density are also important, according to public-health experts.

However, a slew of scientific studies have found that Covid-19 does not spread as easily outside as it does indoors, owing in part to the fact that virus particles disperse more widely in the open air. Some studies have also suggested that exposure to sunlight may render the virus inactive. A systematic review of studies on transmission conditions published in February by the Journal of Infectious Diseases discovered that the chances of SARS-CoV-2 indoor transmission were nearly 19 times higher than those of outdoor spread. According to a December study, 94 percent of super-spreader events in the United States occurred in low-ventilation indoor environments.