The overall pattern of the coronavirus pandemic so far has been a series of COVID-19 waves: surges in new cases followed by declines.

Why does this happen? What will the future hold? Lisa Maragakis, M.D., M.P.H., an infectious disease specialist at Johns Hopkins Medicine, sheds light on what we know now about COVID-19 in communities and why the number of infections increases during certain times.

COVID waves: What causes a spike in coronavirus cases?

So far during the pandemic, several factors have had an impact on whether new COVID-19 cases are increasing or declining in particular locations. These factors include the effectiveness of vaccines over time, human behavior, infection prevention policies, changes to the coronavirus itself, and the number of people who are vulnerable because they have not developed some immunity, whether from natural infection or through vaccination.

For instance, a large spike in U.S. COVID-19 cases occurred over the winter months of 2020–21 when people traveled and gathered for the winter holidays. The arrival of FDA-authorized vaccines in December 2020 helped bring new infection levels back down in many areas through the spring of 2021.

Another surge began in July 2021 as the contagious delta variant began to circulate and eventually become dominant. Waning immunity and relaxation of public policies and infection prevention measures also played a role.

Following COVID-19 precautions, such as getting vaccinated for the coronavirus, practicing physical distancing, hand-washing and mask-wearing, helps to keep viral transmission lower. Cases tend to rise in areas where:

• Fewer people are vaccinated, which means a large number of people are vulnerable to infection.
• Fewer people are wearing masks.
• More people are gathering indoors to eat, drink, celebrate and socialize without physical distancing.

Also, places where people live or work closely together (multigenerational households, long-term-care facilities, prisons and some types of businesses) tend to see more spread of the coronavirus.

Is the delta variant causing more COVID cases?

Yes. Since the beginning of the COVID-19 pandemic, SARS-CoV-2 — the coronavirus that causes COVID-19 — has mutated (changed), resulting in variants of the virus. One of these is called the delta variant. The delta coronavirus is considered one of the most contagious variants so far. Learn more about coronavirus variants.

Although vaccines afford very high protection, breakthrough coronavirus infection with the delta and other variants remains possible. Fortunately, vaccination, even among those who acquire infections, is very effective at preventing serious illness, hospitalization and death from COVID-19.

Tracking Coronavirus Surges: Explaining the Delay

There is a delay between a policy change such as mandating vaccines or relaxing safety measures and when the effects of this change show up in the COVID-19 data. Likewise, in the case of a coronavirus variant, it may take time for the mutated virus to spread, depending on how contagious it is. A very transmissible variant such as delta can cause a faster acceleration in new cases.

An increase in the number of COVID-19 cases or hospitalizations is typically not seen until weeks after a policy or behavior change occurs.

When a person is exposed to the coronavirus, it can take up to two weeks before he or she becomes ill, gets tested and the case is counted in the data. It takes even more time for additional people to become ill after being exposed to that person, and so on. Several cycles of infection must occur before a noticeable increase shows in the data that public health officials use to track the pandemic.

Will there be another COVID-19 surge?

In January 2021, after the FDA had authorized COVID-19 vaccines for emergency use the month before, people in the U.S. began to be vaccinated. By March, the daily number of new infections had steeply declined, and April through June saw those numbers go down even further. But by July, arrival and spread of the delta coronavirus variant sparked another surge in cases of COVID-19.

As of October 2021, infection rates appear to be declining, probably in part due to more people getting vaccinated. More ups and downs in infection rates could be in our future until the pandemic recedes.

Doctors, clinics and hospitals recognize that more COVID-19 surges are possible. They are working with manufacturers to stock up on equipment, and they are continuing their policies for protecting patients and staff members.

Here’s what you can do now:

• Get vaccinated for COVID-19. Learn more about the safety and effectiveness of the COVID-19 vaccines.
• Continue to practice COVID-19 precautions, such as physical distancing, hand-washing and mask-wearing, especially in indoor, public places.
• Check the CDC’s COVID-19 transmission map, and stay in touch with local health authorities, who can provide information if COVID-19 cases begin to increase in your city or town.
• Make sure your household maintains two weeks’ worth of food, prescription medicines and supplies.
• Work with your doctor to ensure that everyone in your household, including children, is up to date on vaccines, including this year’s flu shot.