Compared to other common respiratory viruses, COVID is weird. Most respiratory viruses peak once a year in the winter. COVID peaks twice a year, once in the summer and once in the winter. The early indicators are signaling that this summer’s COVID season is just starting to ramp up. But how will it compare to previous years?
The strange epidemiology of COVID
Since the beginning of the pandemic, there has been an annual surge in COVID cases and deaths every summer in August or September, The graph above shows deaths due to COVID reported by the CDC. We do not see the same phenomenon with other respiratory viruses like influenza. The reasons for this unusual biannual surge in COVID is not entirely clear but may be due to a confluence of several factors including:
- Waning immunity. Both immunization against COVID and infection by COVID will provide partial immunity against future infection. But neither is 100% protective and the immunity tends to wane over time. Similar to influenza vaccinations, the realistic goal of COVID vaccinations is not to prevent everyone from getting infected, it is to prevent as many people as possible from dying of an infection. COVID vaccination is extremely effective in reducing (but not completely eliminating) a person’s chance of dying from a COVID infection but not so great in preventing mild infections, especially after about 6 months following the vaccination.
- COVID mutates easily. Every time the COVID virus mutates, it becomes more resistant to the antibodies in our bloodstream created by either vaccination against previous mutations or infection by previous mutations. One of the reasons that the COVID virus mutates so quickly is that there is just so many cases of COVID infection out there. This equates to more total number of viruses on Earth at any given moment. Statistically, the larger the number of viruses, the greater the opportunity for mutation. In addition, coronaviruses as a group mutate especially easily compared to many other types of viruses.
- Air conditioning. Closing up the house and turning on the AC doesn’t cause COVID, but when it gets hot outside, people tend to go indoors to cool off. It is the close contact that people have with each other in air-conditioned buildings that allows the COVID virus to be easily transmitted from one person to another.
- Summer travel. Travel also promotes close contact with other people in confined spaces with limited ventilation. Cars, airports, and airplanes bring lots of people in proximity, breathing each other’s air. Many people who would stay home from work or school if they are feeling a bit poorly are not willing to cancel their summer car trip or airplane trip to the beach thus increasing the chance that they will infect others.
What happened with COVID cases last year?
In the beginning of the pandemic, it was easier to track the number of cases of COVID. The only places you could get COVID tests were in hospitals or approved outpatient testing sites. All of these reported their results to local health departments that in turn reported to the CDC. Now, many people with mild respiratory infection symptoms do not seek testing. Others do home COVID tests themselves that are not reported to health departments. Consequently, we do not know how many people get new infections. Instead, we have to infer epidemiological trends using other metrics such as wastewater surveillance, the percentage of COVID tests reported to health departments that are actually positive for COVID, and the percentage of emergency department visits that are because of COVID infections.
Wastewater. The graph below shows the amount of COVID virus detected in wastewater monitoring locations over the past year. This shows a large peak in August and a smaller peak in January. The implications is that there were a lot of Americans getting COVID infections in the summer of 2024 – more than in the following winter.
Percentage of ER visits due to COVID. The CDC monitors the primary diagnoses for ER visits. When COVID numbers surge, the percentage of visits to emergency rooms on any given day because of COVID will increase. The graph below shows this data since the beginning of the pandemic. The surges in the percentage of ER visits diagnosed with COVID correlates with the number of hospitalizations as shown in the graph at the top of this post.
The graph below shows this data for the past year and demonstrates a larger number of COVID ER visits in the summer than in the winter. The solid red line depicts the data for all ages combined. Last summer, the highest peaks were seen in infants less than 1 year old (yellow fine dotted line) and the elderly older than 75 (purple line).
Percentage of COVID tests that are positive. Percent test positivity also correlates with surges in the number of COVID cases. Like ER diagnosis percentages and wastewater surveillance, test positivity provides an early warning sign of impending case surges – days or weeks before surges in hospitalizations and deaths. Once again, we see that the percent test positivity was greater last summer than it was last winter.
What about COVID hospitalizations last year?
The winter of 2021 – 2022 (green line below) saw the greatest rate of COVID hospitalization of the pandemic. Since that time, the number of hospitalizations have been falling, due primarily to the availability of vaccinations and population immunity from previous infection. The peak in hospitalizations in the summer of 2024 (light red line below) was slightly greater than the peak in hospitalizations the following winter (light blue line below).
There has also been a slow change in the demographic of COVID patients who get hospitalized. The graph below shows the ages of patients hospitalized since the start of the pandemic. Early on, a high percentage of hospitalized COVID patients were in the 18-49 age range (blue) and 50-64 age range (light green). Now, the vast majority of those hospitalized are over 65 (red). This change in demographic is also likely due to population immunity with most Americans having partial immunity from either vaccination or previous infection with the result that the most medically vulnerable (the elderly) now account for most hospitalizations.
How does COVID now compare to other respiratory infections?
The graph below shows hospitalizations caused by the common respiratory viruses last winter. The number of hospitalizations for COVID (red line) was similar to the number of hospitalizations for RSV (green line). There were many more hospitalizations for influenza (blue line) than for either COVID or RSV. Once again, population immunity from COVID vaccinations and previous COVID infections has resulted in COVID causing less severe infections than earlier in the pandemic. The implication is that influenza has regained its crown as the most epidemiologically significant respiratory infection. So, if your patient will only allow you to give them one vaccination this year, give them a flu shot.
The early signs indicate COVID cases are again increasing
Drilling down on the three early predictors of COVID case surges, there is evidence that the summer surge for 2025 is now beginning:
Wastewater. The graph below shows a slight upward trend in the amount of COVID virus found in wastewater over the past 45 days. The national trend is the black line in the middle, western U.S. is green, southern U.S. is purple, northeastern U.S. is blue, and midwestern U.S. is gold.
Percentage of ER visits due to COVID. The graph below shows COVID deaths (blue bars) and the percentage of ER visits diagnosed with COVID (gold line). The peak in ER visits occurs about 2-3 weeks before the peak in deaths. There has been an uptick in COVID ER visits over the past month.
COVID test percent positivity. The graph below again shows COVID deaths (blue bars). The gold line shows the percentage of COVID tests that were positive for COVID. The peak in test positivity precedes the peak in deaths by 2-3 weeks. Once again, we see a slight uptick in the curve (gold line) over the past month.
COVID has not gone away…
We all so want to be done with COVID. For most Americans, the pandemic with its resultant deaths, lockdowns, masks, and social distancing are a distant memory. Shortages of goods with resultant inflation has given way to economic prosperity and record highs in the stock markets. But COVID is still out there, just waiting for us to close the windows, turn on the air conditioning, and pack the kids into the car for summer vacation.
If last year is a predictor for this year, then in August we should see another surge in COVID hospitalizations, especially infants and the elderly. We will probably see at least as many COVID cases in August as we see later in the year in December. ER physicians, hospitalists, and primary care physicians should resume testing patients who present with symptoms of respiratory infections. We should all make the public aware that COVID cases are increasing so that infected persons seek medical care early, before those infections get severe enough to require hospitalization or cause death.
July 14, 2025
