We are in the midst of another COVID wave. At the Paris Olympics, masks have made a return as athletes fall ill. In the United States, COVID test positivity has reached 15%, compared to just 3% this spring. President Joe Biden, second gentleman Doug Emhoff, and senator John Fetterman are among the infected. From Canada to Japan, countries still tracking the spread of COVID are reporting that the virus is surging. Fortunately, widespread immunity is reducing severe illness. However, almost five years after first making headlines, COVID seems to be infecting as many people as ever.
Wastewater data indicates that COVID is still widespread. Source: CDC, Biobot.io, @JPWeiland
Is this normal?
Normality is a tricky thing to define. Perhaps the best way is to compare COVID to other viral respiratory illnesses. Currently, the two deadliest in the United States are RSV, which kills more than 12,000 Americans a year, and the flu, which kills almost 40,000 (these are just averages as both can vary from year to year). Comparing them to COVID is not an apples-to-apples comparison, however, especially since there are two flu A viruses (each distinct enough to cause its own pandemic) and one flu B virus. COVID-19, despite having variants, is only one. Even so, it killed almost 80,000 Americans in 2023, easily exceeding all other infectious diseases. In that regard, it isn’t exactly normal.
Additionally, whereas flu and RSV hospitalizations only surge every winter, COVID-19 has multiple peaks each year. Even in the hottest days of summer, the virus can hospitalize more people than RSV and flu do during the peak of winter. Additionally, after major outbreaks, COVID hospitalizations remain relatively elevated, whereas RSV and flu practically disappear. It’s harder to say this about other respiratory viruses that aren't tracked as well, but test positivity indicates even other coronaviruses do not behave this way.
COVID hospitalizations have fallen but remain unusually high compared to what’s seen with other respiratory viruses. Source: CDC COVID-NET
Why is COVID like this?
COVID is not behaving like its peers, even though almost five years have passed since its emergence. That makes it increasingly unlikely that its unusual behavior is due to it being a new virus, and more due to factors inherent to the virus. In fact, evidence does seem to support this.
One factor is that SARS-CoV-2, the virus that causes COVID-19, is simply more contagious. This is measured by a value called the basic reproductive number, or R0: the average number of people a sick person will infect in a susceptible population. For the flu, that number is around two. For RSV, it’s around three. For the Omicron variant of SARS-CoV-2, it’s over eight. This puts it much closer to viruses like Smallpox, Polio, and Measles than the flu, but immunity to those viruses is also lifelong. That puts SARS-CoV-2 in a league of its own.
To understand why this matters, we must revisit a concept often cited in early months of the pandemic: herd immunity. Essentially, viral outbreaks stop as they run out of susceptible people to infect. Herd immunity is the threshold of how many people need to be infected or made immune in some other way (like vaccination) to slow an outbreak.
Immunity to viruses like the flu and SARS-CoV-2 is temporary, so there is no permanent herd immunity. Instead, they surge, slow, and surge again as soon as enough people lose immunity. The more contagious a virus is, the lower the herd immunity threshold, meaning that outbreaks are much more frequent. This can be demonstrated by a simple SIR model (a common disease model used since the 20th century). We see that the less contagious virus behaves like the flu with pronounced lulls and sharp peaks, whereas the more contagious virus is always around.
Another factor is that SARS-CoV-2 evolves faster. As the virus changes, it can become more contagious or unrecognizable to antibodies produced by the immune system. Though it’s not a rule, this usually means that the faster a virus evolves, the more people it can infect.
Each year, SARS-CoV-2 substitutes an average of 0.02 amino acid substitutions per residue in the S1 region of its Spike protein. S1 is a crucial part of the virus that affects how the virus attaches to cells and escapes immunity, so mutations here are particularly impactful. Compared to similar parts of other viruses, this rate of evolution is almost ten times faster than what’s seen with other coronaviruses and even faster than H3N2, the most dangerous seasonal flu strain. Once again, COVID is in a league of its own.
SARS-CoV-2 seems to be the fastest-evolving respiratory virus on the market. Source
Even as the world recovers from COVID, the virus's ability to infect remains robust. Repeated waves of infections and hospitalizations will continue. While deaths have fallen, poorly understood consequences like long COVID remain a persistent threat.
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