DukeMedicine HealthNews: What have we learned about how COVID-19 spreads?
Nicholas Turner, MD, MHSc: Novel coronavirus poses a few challenges. Compared to other respiratory viruses (including SARS from 2003), it has a higher transmission rate, can be spread while symptoms are still mild or have yet to develop, and may be spread by children even with minimal or no symptoms.
Epidemiologic modeling studies suggest cases with mild or minimal symptoms may contribute significantly to spread, which makes it very hard for quarantine to halt the virus when many of these cases could go unrecognized. China serves as an example here: Even despite the strictest of quarantine measures and travel restrictions deployed on a large scale, it was not possible to completely halt the spread of coronavirus.
DMHN: Do you think self-quarantine measures will be effective?
Dr. Turner: We are realistically in a risk-mitigation phase. By asking symptomatic individuals to self-quarantine and by preventing gatherings of any more than 10 people (based on current Centers for Disease Control [CDC] guidelines, which notably may change day to day as the situation evolves), it should be possible to both slow the spread of the virus and reduce the total number of cases likely to occur overall.
One of the most important risk-mitigation goals is also to reduce coronavirus exposure for some of our highest risk populations who suffer more severe illness when they contract coronavirus (e.g., the elderly or those with chronic medical conditions).
DMHN: We see that not everyone is abiding by the CDC recommendations at this point. Could that make a substantial difference in the length of time we can expect to see new infections?
Dr. Turner: If symptomatic individuals were to ignore requests for self-quarantine or if group gatherings were to continue as usual, we would anticipate both larger case numbers and a longer total duration of the pandemic. Large gatherings have already shown to provide the perfect situation for coronavirus to spread: In one current cluster from Massachusetts, more than 70 of 92 identified cases were associated with one conference.
Beyond just larger case numbers, rapid transmission due to large group gatherings can put a huge strain on communities. It can be hard for healthcare systems, or really any service industry, to remain operational if a significant fraction of the workforce is out sick all at the same time.
DMNH: Can we look to other countries for insight into our own timeline?
Dr. Turner: It is harder than it seems to compare timelines between different countries. A lot of variables play into the trends: the population’s age distribution, medical conditions, density, availability of testing, what prevention measures have been enacted, etc. That said, a key tipping point seems to be when the first cases of local community transmission begin to occur. I would anticipate the sharper rise in cases to occur within the one to two weeks following the first detected local transmission events.
DMHN: Does the idea that heat will curb infections hold any water?
Dr. Turner: To tease out the temperature issues a bit, it helps to think about viral survival. From a strictly physical standpoint, temperatures required to kill or inactive viruses are often higher than 60 C (140 F), well above typical environmental temperatures. Although seasonal temperature changes are insufficient to outright kill coronavirus, there are quite a few similar viruses that tend to survive for longer on surfaces in drier, cooler weather, which may explain why some viruses do develop seasonal trends.
I think it’s too soon to tell if COVID19 will show seasonal patterns, but I wouldn’t be surprised if it did, given how similar viruses behave.
DMHN: We’re hearing about the importance of flattening the curve. While that reduces the number of people who get sick at one time, does it affect the overall number of people infected by the end of the outbreak?
Dr. Thomas: With a flattened epi curve, it’s still possible for the same total number of people to become sick over time, but the huge benefit is that infrastructure (for healthcare or society in general) isn’t pushed to its breaking point. The most severe strains on healthcare and other services occurs when attack rates from an infection leave large proportions of a population sick all at once.
Steep epi curves can overwhelm hospitals for their bed capacity, lead to medication shortages, or simply leave too many staff ill to be able to care for others. Public health officials use tools like social distancing and restrictions on large group gatherings to flatten the curve and prevent health care needs from exceeding capacity at any given point in time.
DMHN: Does existing information shed any light on whether we can expect to see a resurgence of cases in the fall?
Dr. Thomas: There are certainly other circulating coronaviruses that already show a seasonal trend, and I think the tendency for COVID19 to spread efficiently even in milder cases and among minimally symptomatic children means it’s pretty well suited to become another seasonal virus – it’s just too soon to tell for certain.