September 27, 2021

7 reasons why Covid-19 cases keep surging—and falling

Daily Briefing

    While recent Covid-19 surges have often been attributed to low vaccination rates or loosened safety restrictions, public health experts suggest other factors may also contribute to the virus's rises and falls, German Lopez reports for Vox.

    How much worse will the 'delta surge' get? Watch these 7 factors.

    Florida's 'mysterious' summer Covid-19 surge

    Toward the end of the summer, Florida became the center of the country's recent Covid-19 surge, reporting more hospitalizations and deaths than any other state. However, according to Lopez, health experts are still uncertain about what caused the state's outsized surge compared to the rest of the country.

    In general, many coronavirus outbreaks in southern states over the summer were attributed to low vaccination rates in the region, which typically correlate to increased Covid-19 cases and deaths, Lopez reports. Currently, 7 of the 10 states with the lowest vaccination rates are in the South.

    However, Florida is not one of these states, currently ranking 20th in the nation, with 56% of its population fully vaccinated—slightly above the national average. And during the peak of the state's Covid-19 surge in mid-August, around 51% of Florida's population was fully vaccinated, matching the national average at the time.

    Another potential explanation for Florida's surge is its more relaxed public safety measures, but Lopez writes that it is unclear if these loosened restrictions actually changed how people behaved.

    For example, based on mobility data from Google, Florida residents were 14% less likely to go to retail and recreational outlets in mid-August than before the pandemic, which was similar to the data for California residents and lower than New York residents. In addition, Carnegie Mellon University's COVIDcast research found that Florida residents were more likely to wear a mask than residents of New York. However, neither California nor New York saw surges as large as Florida's in August.

    This information led Lopez to conclude that Florida's summer Covid-19 surge cannot be explained "solely [due to] reopenings and vaccinations."

    Why Covid-19 surges and falls, according to experts

    According to Lopez, public health experts are still uncertain about why the prevalence of coronavirus seems to rise and fall, but they pointed out several common factors that could contribute to these cycles, including:

    • Immunity: When more people have immunity against Covid-19—whether through vaccination or infection—the virus is no longer be able to spread as quickly or cause severe illness as easily. According to Lopez, state vaccination rates likely underestimate the true immunity in the community since they don't take natural immunity into account.
    • Public safety measures: Higher rates of masking and social distancing help reduce the risk of outbreaks occurring. Therefore, the introduction and cessation of safety precautions may also contribute to the rise and fall of Covid-19 surges. Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security, said people relax when cases are low, but then they "respond to [a surge] and tighten up protective measures for a while. And then cases go down, and people relax again."
    • New variants: If new variants emerge, particularly those that are more transmissible or evade previous immunity, surges are more likely to happen. This already happened with the highly infectious delta variant, Lopez writes, which has caused Covid-19 surges around the world.
    • Weather: Poor weather conditions—such as heat, rain, cold, or snow—can push people to stay indoors, where the virus can spread more easily due to poor ventilation. According to Lopez, upcoming cold weather during the fall and winter could lead to more cases in northern states as people stay inside to keep warm.
    • Geographic differences: Masking and vaccination rates are typically not distributed equally in a state, Lopez reports. There may be concentrated pockets with low vaccination rates or where people are less cautious, and surges in those specific areas may make it seem like there is a surge statewide. "States are too big, and you want to look at towns and communities," said Ashish Jha, dean of the Brown University of Public Health.
    • Biological factors: Age and comorbidities, such as asthma and obesity, increase the risk of dying from Covid-19. Areas with large senior populations, more people with health conditions, or both could experience larger outbreaks due to their increased risk.
    • Chance: "Sometimes, circumstances align in a way that's simply unfortunate," Lopez writes. For example, a new variant may emerge right as an area is fully reopening, causing rapid spread when precautions are the most relaxed.

    Although there is still uncertainty around what specifically causes Covid-19 surges, Lopez writes that being aware of these potential factors is important, especially since some, such as vaccination and safety precautions, are within people's control and can help prevent new surges.

    "It might not get us all the way to the finish line—at least until far more people are fully vaccinated," Lopez writes, "but taking advantage of these things we do have control over can at least reduce the damage until the pandemic ends." (Lopez, Vox, 9/23)

    How much worse will the 'delta surge' get? Watch these 7 factors.

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    Read the latest take

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