The World Health Organization and other international health organizations often refer to "waves" of a pandemic, but no formal definition exists. A wave refers to a a rising number of COVID-19 cases that has a specific peak and then declines. On a graph it looks similar to the shape of a wave that increases, tops out, then decreases. A wave can also be referred to in some cases as a surge or outbreak.

Public health scientists first began using this term to describe different peaks and valleys of infections during influenza outbreaks in the late 1800s and the 1918-1929 "Spanish flu." We use this historical pandemic to help understand and categorize disease spread and use findings as models that might help us predict how COVID-19 will act in modern times.

Each wave has a different feature and can impact different populations, even within the same country. This depends on if the disease is seasonal or if it is a particularly distinct illness like H1N1 in 2009-2010, which impacted older people with underlying illnesses more than other populations. When people are outside in the summer, waves of illnesses for influenza usually decrease because people aren't in school or forced to be inside due to the weather.

Waves can improve dramatically by reaching levels of immunity in human populations. As more people become immune to COVID-19 and the spread of it stops, the virus eventually loses people to infect.

The first wave of an epidemic means that that virus starts spreading among many people, more and more each day, and then a peak number of daily cases is reached. During this time, positive COVID-19 tests for people with and without symptoms, case numbers, hospitalizations, and deaths are also increasing. After that peak, the virus spreads less as more people have either been infected or have learned how to prevent themselves from getting infected or spreading the virus, so rates of transmission get lower and fewer people are getting sick over that period of time.

The second wave occurs when daily case numbers begin increasing again. This wave tends to be worse than the first in terms of daily infections and severe illnesses. The term "second wave" is used so the public is able to distinguish it from the first surge in cases. It marks a period of time when the virus' spread begins a sustained rise in infections. Often treatments and vaccines start to be tested during this period of time because scientists hopefully know more about the virus, its genetics, how it spreads, and what other impacts on the body it has during this time. During the second wave doctors are typically able to diagnose the illness more easily.

The third wave occurs when a third peak is observed in a population and often occurs as a result of the social determinants of health. This means that people with low-incomes, People of Color, people without homes and healthcare, and vulnerable populations are impacted because they are not able to avoid working in high-exposure jobs like grocery stores, airports, and restaurants. This wave often worsens economic and healthcare inequities because people cannot take time off of work when sick, causing others to become infected and making the cycle continue. Vaccines might begin to roll out during this period, helping populations work toward herd immunity to prevent future waves from occurring or being as severe.

Of course, all of these waves are caused by different human behaviors, a lack of governmental actions and rules, travel, daily activities, viral variants, and more. Waves occur at different times in different regions and look different in each place. Factors include the number of people living close together, whether people have access to running water and medical care, and if they are able to use prevention measures.

As COVID-19 has spread across the world, nations are reaching the first, second, and third waves of the epidemic at different times. This is usually marked by specific aspects of each region and its people, but is a complex topic and cannot be sharply defined.