When Nevada Governor Steve Sisolak announced the momentous closure of all nonessential businesses in the state on March 18, he repeatedly issued a stern warning to heed social distancing protocols. During his 18 minute address, the Governor cited a coronavirus study published the day before that offers a sobering assessment of the rate at which the COVID-19 virus will spread without a vaccine or current social distancing policies.
Even with taking the most comprehensive “non-pharmaceutical interventions,” the scientific need to maintain current virus suppression policies will likely endure until a vaccine is developed and distributed, which according to the study’s authors, could be 18 months or more.
Imperial College researchers say COVID-19 is the most serious since the 1918 influenza pandemic that infected one third of the planet’s population and killed some 50 million people worldwide, 675,000 in the United States.
“The World Health Organization has stated that the numbers of secondary infections generated from one infected individual is between 2 and 2.5 for COVID-19,” said Governor Steve Sisolak in an address given on March 18 that closed all nonessential businesses in the state. “That’s higher than the rate of reproduction for the flu. And according to experts at Imperial College in the United Kingdom, the doubling rate of this virus globally is roughly four to five days. That spread is truly alarming. Our hospitals are prepared right now to help those most critically affected by this virus.”
When asked about the role the Imperial College study played in the Governor’s decision, spokesperson Ryan McInerney wrote in an email that “the Governor consulted a variety of scientific sources, including the Imperial College study, and opinions from medical experts to make his decision.”
In his address on the 18th, Governor Sisolak said he had consulted with the chief medical officers of all of Nevada’s hospitals for their assessments and most responsible next steps.
“They have advised me that the most effective course of action is to direct all Nevadans to stay home and for non-essential businesses to close to the public for 30 days.”
The coronavirus suppression strategies underway in Nevada largely mirror those outlined in the Imperial College study. Study authors consider two broad courses of action in developing their computer model of potential coronavirus outcomes, mitigation or suppression.
Mitigation focuses on slowing but not stopping the epidemic spread.
According to the Imperial College study, merely enacting mitigation strategies are “unlikely to be a viable option without overwhelming healthcare systems.” Suppression is recommended.
Suppression is intended to reverse epidemic growth and reduce case numbers to “low levels and maintain that status indefinitely.” A policy of suppression is guiding actions being taken in Nevada and across the globe.
The study concludes that, without a vaccine, in order to flatten the transmission growth rate, “a combination of case isolation, social distancing of the entire population and either household quarantine or school and university closure are required.”
There is no vaccine for the COVID-19 virus, and the Imperial College study authors postulate it will take “18 months or more” before one is widely available. In that context, study authors conclude, a program of suppression will have to be maintained to prevent the spread of the virus.
According to the World Health Organization, once a new strain of influenza virus is identified, it typically takes five to six months for the first round of tested vaccine to become available.
Historically, vaccines have effectively arrested transmission rates during influenza pandemics. A good example is in 2009 when the Swine Flu killed between 151,700 and 575,400 people worldwide with 12,469 deaths in the United States, according to the United States Centers for Disease Control.
Adding uncertainty to the timeline of global recovery from COVID-19, Imperial College researchers note that there is no guarantee initial vaccines will work. From the Imperial College report:
“The major challenge of suppression is that this type of intensive intervention package – or something equivalently effective at reducing transmission – will need to be maintained until a vaccine becomes available (potentially 18 months or more) – given that we predict that transmission will quickly rebound if interventions are relaxed. We show that intermittent social distancing – triggered by trends in disease surveillance – may allow interventions to be relaxed temporarily in relative short time windows, but measures will need to be reintroduced if or when case numbers rebound. Last, while experience in China and now South Korea show that suppression is possible in the short term, it remains to be seen whether it is possible long-term, and whether the social and economic costs of the interventions adopted thus far can be reduced.”
That the Imperial College researchers assert that the coronavirus rebounds when suppression measures are relaxed without the support of a vaccine has ignited a debate among scientists. How long the virus will actively affect humanity is at issue. The Imperial College authors say that based on recent observations in China and South Korea, COVID-19 comes back when not held in check.
Researchers at the New England Complex Systems Institute say the Imperial College study is important and should be used to inform lawmakers because it accurately correlates both the social/governmental response and the contagion, but they pointed out a few mistakes.
“They ignore standard Contact Tracing allowing isolation of infected prior to symptoms. They also ignore door-to-door monitoring to identify cases with symptoms. Their conclusions that there will be resurgent outbreaks are wrong. After a few weeks of lockdown almost all infectious people are identified and their contacts are isolated prior to symptoms and cannot infect others. The outbreak can be stopped completely with no resurgence as in China, where new cases were down to one yesterday, after excluding imported international travelers that are quarantined.”
How long the transmission rate will be in ascent is unknown.
The Imperial College study results are based on a computer model that takes into account factors such as incubation period, population densities, and perhaps most important for public policy, methods of transmission. The model incorporated the following methods of transmission in its assessment, which are in many ways reflected in public policy.
“Transmission events occur through contacts made between susceptible and infectious individuals in either the household, workplace, school or randomly in the community, with the latter depending on spatial distance between contacts. Per-capita contacts within schools were assumed to be double those elsewhere in order to reproduce the attack rates in children observed in past influenza pandemics. With the parameterisation above, approximately one third of transmission occurs in the household, one third in schools and workplaces and the remaining third in the community.”
The choices facing civic leaders are stark. The economic impacts of virus suppression policies are unknown in gory specifics, but certainly they are not good. During his recent announcement to close nonessential businesses, Governor Sisolak said the decision was based on saving lives.
“This is affecting the lives of our citizens. People are dying. You know, every day that is delayed here, I’m losing a dozen people on the back end that are gonna die as a result of this.”
For more, see the Imperial College study: Impact of non-pharmaceutical interventions (NPIs) to reduce COVID19 mortality and healthcare demand.