NRPLUS MEMBER ARTICLE E arly in the COVID pandemic, there was much discussion of super-spreaders — a single individual attending a routine and harmless event such as a birthday party, church service, or dinner party who caused many other people to become infected. As the pandemic has evolved and started significantly infecting young adults, we have become more focused on mass-gathering places such as bars and beaches.
Both super-spreaders and places for mass gatherings share something in common: multiple individuals coming into contact with multiple other individuals in a relatively short period of time. This type of mixing pattern is termed “concurrency” and has been used to explain how many infectious diseases such as HIV spread through a community. A correlate of this model is that in settings where individuals have high levels of concurrency, there is the opportunity for them to infect more than one person, causing exponential growth in the infection (e.g., an outbreak). In contrast, without concurrency, one person may not infect anyone else and the spread slows.
However, concurrency alone does not lead to continued transmission or outbreaks of COVID. Other factors need to be present. Public-health and government officials need to take the combination of factors beyond concurrency into account when making recommendations about events and settings. If one dives deep into the White House’s plan for the phased reopening of the country and watches the current policy reactions of government officials to the emerging hotspot, the influence of this multifactorial perspective on the growth in COVID infections is apparent.
The risk of transmission of an infectious disease such as COVID is a function of the likelihood that an infected person comes into contact with an uninfected person, the efficiency of spread from person to person, and the period of time during which the infected person is best able to transmit.
The probability of contact is related to the number of contacts a person has in a short period of time and the percentage of people who are infected in the area. The best measure of percentage infected is the percentage of tests that are positive — positivity. It is reasonable to say if the positivity is below 5 percent, the probability of contact is lower, and if the positivity is above 10 percent, then the area is a hot spot and probability of contact is higher.
With regard to efficiency of spread, COVID spreads when aerosolized droplets of a sufficient quantity of the virus move from one person to another. The quantity is in turn related to how forcefully one is breathing/coughing/speaking, the distance between contacts, whether there is some protective equipment (such as facial covering) that blocks airflow between them, and whether local air flow disperses the stream of droplets or focuses the flow of droplets. Finally, the amount of COVID virus capable of being transmitted is greatest early in the infection, before significant symptoms develop and social interactions may slow.
Concurrency is a combination of these factors — number of contacts while most contagious. If person A, who has recently become infected by person B, comes into contact with person C soon after being infected, it is said that A has concurrent contacts (B and C) and that because of A’s concurrent contacts, C is at risk for becoming infected by A. Further, infection spread will accelerate if A has more concurrent contacts than just B and C. C’s risk diminishes if B was not infected, if A and C did not have close direct face to face contact, and if the time when A and C came into contact was after A’s viral load dropped to a level where A was not contagious.
As such, if the positivity of COVID is low enough or the number of contacts during the time one is contagious is limited, then the likelihood of transmission is low. But, if positivity is high in the community and high levels of concurrency are occurring because of a gathering of people in a social or professional context, then other factors must be examined to determine whether the setting or type of gathering will result in a super-spreader or outbreak event.
Settings where there is direct face-to-face talking without facial coverings, such as bars or large indoor dinner parties, are at high risk for a super-spreader or outbreak event. Mass transit, museums, theaters, outdoor sporting events, and maybe even beaches don’t inherently create close direct face-to-face contact and thus may not represent high risk for accelerated transmission. However, in any of these settings, if people engage in direct face-to-face contact with people outside their own household without a facial covering, then risk of transmission for an outbreak goes up.
Over the next one to two years, as we live with the persistent ebb and flow of the virus, we need to differentiate social and commercial settings that may result in sporadic infections and those settings associated with exponential growth of infection, i.e., super-spreaders and outbreaks. We need more investigations of the disease to confirm this thinking, but until then, we need to at least begin to differentiate between individual risk and risk for outbreaks as we think about public-health policy.