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Florida’s mysterious Covid-19 surge

We still don’t have all the answers for why Covid-19 cases rise and fall, but there are some explanations.

Two medical workers in silhouette at an outdoor Covid testing facility with palm trees in the background.
Medical workers wait for a vehicle at a drive-through Covid-19 testing site on July 24, 2020, in St. Petersburg, Florida.
Bryan Smith/AFP via Getty Images

Toward the end of the summer, Florida became the epicenter for America’s recent Covid-19 wave — reporting more hospitalizations and deaths than any other state in the country. But there was and still is surprisingly little certainty, among experts, over one question about Florida’s surge: Why did it happen?

The most common explanation for the outbreaks in the South that we saw over the recent summer was the low vaccination rates across the region. It’s true vaccination rates are low across the South: Seven of the 10 states with the lowest vaccination rates are in the region. And lower vaccine rates do correlate with more Covid-19 cases and deaths.

But Florida defies the regional trend. The state ranks 20th for full vaccination in the US, with 56 percent of people fully vaccinated — not great, but a little above the national rate. At the peak of its outbreak in mid-August, Florida had fully vaccinated about 51 percent of its population — again, not great, but in line with the national rate.

Maybe Florida loosened restrictions too quickly and more aggressively? It’s certainly true that Gov. Ron DeSantis has taken a more hands-off approach than leaders in blue states, but it’s not clear if this actually led to differences in how the public behaved.

According to Google’s mobility data, Floridians around mid-August were about 14 percent less likely to travel to retail and recreational outlets compared to pre-pandemic times. That’s almost the same as Californians, and actually lower than New Yorkers. Neither New York (about 59 percent fully vaccinated at the time) nor California (about 54 percent fully vaccinated at the time — not much higher than Florida) saw surges anywhere as bad as Florida’s in August.

The same trend holds for other metrics that measure precaution. Based on Carnegie Mellon University’s COVIDcast, through August, Floridians were more likely to mask up than New Yorkers or residents in other states that didn’t see nearly as big Covid-19 surges.

Based on OpenTable’s restaurant reservation data, Florida was back to pre-pandemic numbers for restaurant reservations around mid-August, but that wasn’t too different from the US as a whole. Some states, like New Jersey and Connecticut, equaled or surpassed their pre-pandemic baseline for restaurant reservations and didn’t see anywhere near the surge that Florida did (although both benefited from significantly higher vaccination rates than Florida).

This isn’t to say that nothing matters in the fight against Covid-19. We know vaccines work to protect people from severe illness, including against the delta variant. Social distancing, masking, and restrictions do, too. Chances are Florida’s surge would have been much smaller if it had done better on all these fronts.

But Florida’s example complicates any story of recent Covid-19 surges that focuses solely on reopenings and vaccinations. Something else seems to be going on, and experts aren’t totally sure what. “There are things that, to be honest, we don’t fully understand,” Ashish Jha, dean of the Brown University School of Public Health, told me.

We don’t know everything about why Covid-19 cases rise, and we don’t know everything about why they fall, either. David Leonhardt and Ashley Wu at the New York Times recently demonstrated that the coronavirus appears to follow two-month cycles in its rises and falls.

Yet, experts told them, it isn’t clear why. “We still are really in the cave ages in terms of understanding how viruses emerge, how they spread, how they start and stop, why they do what they do,” Michael Osterholm, an epidemiologist at the University of Minnesota, said.

Experts point to some possible factors that contribute to trends in Covid-19 — widely discussed ones like vaccination and precautions, but also less covered issues like the weather, geographic concentration, and luck. But they acknowledge that there could be something going on that we just don’t know of or understand yet.

Figuring out all of this is crucial: It could be the difference between enabling and preventing not just the continued spread of Covid-19 but perhaps the next pandemic, too.

What we know about the spread of Covid-19

In my conversations with experts, several common factors came up when explaining why the coronavirus rises and falls:

Vaccination and natural immunity: When more people have immunity built up against Covid-19, by natural or vaccine-induced means, the virus is going to hit a wall in which it can no longer spread — or at least cause serious illness — much more quickly.

One tricky issue here is vaccination rates at the state level likely understate community immunity, since they can’t capture — and no data does a great job capturing — natural immunity. When looking at a state like Florida, perhaps its level of immunity is lower than vaccination numbers suggest because of the missing puzzle piece of natural immunity.

Precautions around the virus: If people are mingling closely in indoor spaces without masks, they’re much more likely to spread a respiratory pathogen like the coronavirus. So whether they’re enforced by the government or voluntarily taken up by the public, higher rates of masking and social distancing help keep outbreaks away. Rapid testing or better ventilation could help, too, although neither has been widely adopted in the US.

Some of this is cyclical. When cases are low, people relax on precautions, Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security, told me. “And then they respond to [a surge] and tighten up protective measures for a while. And then cases go down, and people relax again.” Less caution during better times may make sense from an individual perspective, but it also could help contribute to the cyclical nature of Covid-19.

Variants: New versions of a virus that are more transmissible or evade previous immunity are more likely to cause surges. That’s been very apparent with delta, as it has caused spikes in cases, hospitalizations, and deaths in places ranging from the UK to India to the US.

Weather: There are two main mechanisms at play here: First, heat, humidity, and the open air seem to make it harder for the virus to survive and spread. Second, cold, heat, snow, or rain can push people indoors, where the virus has a much easier time spreading due to worse ventilation.

Currently, the latter effect seems more important, especially in places like Florida where the heat and humidity, as well as summer rains, ultimately get people to go indoors instead of benefiting from the weather’s virus-killing effects outside. This coming fall and winter, the cold could also propagate more spread in northern parts of the country as people go inside to warm up.

Geographic concentration: Even when a place appears to be doing well on masking or vaccination rates, that might not be distributed evenly; there may be concentrated pockets where people aren’t following precautions or getting vaccinated. That could lead to what looks like a surge in statewide data but is by and large concentrated in specific hot spots.

In Florida, for example, the state’s above-average vaccination rate masks that a handful of counties, particularly in more rural and conservative regions, still have vaccination rates below 40 percent. And those counties and surrounding areas generally have more Covid-19 cases. “States are too big, and you want to look at towns and communities,” Jha said.

Age, comorbidities, and other biological factors: We know that older people are more likely to die of Covid-19, as are people with comorbidities such as asthma, obesity, or neurological conditions. There are also likely other biological issues, down to genetics, that aren’t yet confirmed but could play a role, too. So if a place has an older population (as is true for Florida), or more people with conditions that make them vulnerable to Covid-19, or both, it could see a bigger outbreak.

Luck: Sometimes, circumstances align in a way that’s simply unfortunate. Perhaps someone who ended up spreading the coronavirus to 30 other people in a restaurant wouldn’t have done so if he had gone to the restaurant one day or hours later, when he might not have been as contagious or when fewer people might have been around. Maybe a contagious variant pops up just as a city, state, or country is fully reopening, enabling rapid spread at the exact worst time.

There was one other factor that came up in my conversations with experts: the unknown. Given that we’re still learning, even after a year and a half of this pandemic, about Covid-19 and its spread, no one can offer all the answers.

That might seem like a reason to despair. If we don’t fully understand what causes the rise and fall of pathogens, how can we possibly hope to tame them?

The good news, though, is that we do know some contributors, and we have real control over a few. Policymakers and the public may not be able to do much about the weather, biological factors, or luck, but they can do plenty when it comes to getting vaccinated and following recommended precautions against Covid-19, from physical distancing to masking.

It might not get us all the way to the finish line — at least until far more people are fully vaccinated — but taking advantage of these things we do have control over can at least reduce the damage until the pandemic ends.