We know from past pandemics that secondary waves happen, but what can the world expect with COVID-19?
Michael Oglesbee, director of Ohio State’s Infectious Diseases Institute, explained stay-at-home orders and social distancing reduced the number of COVID-19 cases in the initial surge — flattening the curve, in 2020-speak.
“Recovery from infection can result in a level of protective immunity,” Oglesbee said, “although we don’t know the strength of that immunity, and generally there are not a sufficient number of people having recovered from infection in the initial peak to confer herd immunity — a level of immunity in the population that prevents circulation of the virus.”
Oglesbee discussed possible approaches for if numbers should rise again and how leaders should respond.
In the absence of a vaccine and with continued presence of the virus, continued infections will occur. Relaxed mitigation efforts that result in increased mixing of infected and uninfected people will result in a rise in the number of new COVID-19 cases. If the rise is abrupt, we call this a secondary peak or "wave."
The potential for secondary peaks is a particular concern for COVID-19 because it is difficult to know who is infected and who is not. Fifty percent of infected people can shed the virus without symptoms, and people destined to develop symptoms of COVID-19 are shedding virus before symptoms manifest.
We also do not know exactly how many people have been infected already and may be immune.
Secondary peaks begin as outbreaks localized to specific areas.
For example, if an infected individual participates in a community event where there are no distancing measures, the infection can spread to susceptible individuals at the event. The cluster of infections and the associated disease is known as a "hot spot."
The size of the hot spot is a function of the number of people participating in the event and the person-to-person contact that may have occurred during the event. If infection spreads from the hot spot to the larger population, then we see a secondary peak in COVID-19 cases. The problem is magnified if we have multiple hot spots.
New infections are inevitable. But we can contain outbreaks to prevent them from becoming large hot spots, and we can prevent hot spots from evolving into secondary peaks at the population level.
This is possible when overall incidence of COVID-19 is on the decline.
The approach relies on our ability to test for the presence of virus and, through contact tracing, quickly identify people who have come in contact with infected individuals. We need to limit movement of both the infected individual and those in contact with that person — self-isolation and quarantine.
If we cannot contain growth of the hot spot, then we can resume distancing measures at the local level, thereby preventing a second peak at the level of a city, county, state or nation.
Studies of the 1918 pandemic reveal three factors that contributed to the multiple waves of disease incidence:
- schools opening and closing;
- temperature changes during the outbreak;
- and changes in human behavior in response to the outbreak.
Summer closure of schools helped to reduce influenza disease incidence, and returning to school contributed to secondary peaks. This effect may reflect the enhanced susceptibility of children to influenza infection. With COVID-19, children seem to have an enhanced resistance to infection, making it unclear how school opening and closures will affect incidence rates.
Temperature and humidity help to determine the transmissibility of influenza virus, but it is unknown how environmental conditions affect transmission of the coronavirus that causes COVID-19.
Distancing measures or lack thereof were the primary determinant of the incidence of influenza cases and the same is true for COVID-19.
As distancing measures are gradually relaxed and businesses re-open, we will learn what works and what needs to be improved upon with how people mix in public settings and what we can do to minimize environmental risk of infection to reasonable levels. Considerations include how we clean and disinfect common-use spaces and how we define the number of people who should occupy a given space.
The concept is “balanced risk.”
New infections will occur, and we simply need to be able to effectively monitor the incidence and location of new infections, not simply the incidence of disease. We must have a coordinated public health response to localized outbreaks and a caseload that can be managed by our hospitals and clinics.
The appropriate public health response ultimately requires a commitment of resources by state governments.
We need to be proactive and not simply reactive to subsequent peaks or new pandemics.
We need people who do monitoring, testing and contact tracing and the infrastructure to support data sharing.
We need people that look at the data in real time to forecast the potential for new outbreaks so that monitoring and response teams are positioned to respond.
We need coordination between universities, health care systems and local and state health departments to maximize resources and must have channels through which we can influence policy based upon our assessment of risks and ability to respond.
In the end, we must embrace the understanding that public health requires sustained investment. When our efforts are effective at preventing infectious disease challenges (or secondary peaks), there will be those who say we have overreacted or over-invested. But the price of vigilance is infinitely more cost effective than simply working in response mode — putting out the latest fire.
We are always at risk for new pandemics, be they unfamiliar as in the case of COVID-19, or familiar as in the case of influenza.