While some have expressed hope that the novel coronavirus could diminish in the summer, the report stressed that, of eight major pandemics since the early 1700s, no clear seasonal pattern emerged for most.Seven of the pandemic viruses disappeared without significant human intervention, but then returned in a second substantial peak about six months later.Some pandemics showed smaller waves of cases over the course of the first two years after the initial wave. Only one pandemic, in 1968, followed the traditional, influenza-like season pattern. In some areas, especially in Europe, mortality associated with the pandemic was higher in the second year.
The CIDRAP report posited that a number of key points from the epidemiology of past influenza pandemics could provide insight into the current coronavirus outbreak. First, the pandemic will likely last 18 to 24 months, as herd immunity generally develops. 60% to 70% of the population may need to be immune in order to reach a high enough threshold to stop the pandemic. The WHO warned last week that only 2%-3% of the global population appear to have been infected.
The duration of immunity to natural infection is also still unknown. It could be anywhere from a few months to several years. The World Health Organization (WHO) warned on Saturday that there was currently "no evidence" that people who have recovered from COVID-19 and possess antibodies are protected from a second coronavirus infection.A vaccine could affect the course of the pandemic, but a vaccine will likely not be available until at least 2021 and could be delayed by unexpected challenges, according to the report.
"The idea that this is going to be done soon defies microbiology,” said Mike Osterholm, the director of CIDRAP, to CNN.The report, written by Osterholm, Dr. Kristine A. Moore, Dr. Marc Lipsitch, and John M. Barry, urged governments to inform citizens that the pandemic will “not be over soon” and that they should be prepared for possible periodic resurgences of the virus over the next two years.
The CIDRAP report suggests three possible scenarios that could occur in the future of the COVID-19 pandemic.
The first scenario would see the current wave of the pandemic followed by a series of repetitive smaller waves over a one to two year period, gradually diminishing in 2021. The waves may vary geographically and depend on mitigation measures. These waves could require periodic reinstated of measures such as lockdowns and social distancing regulations.
The second scenario sees the current wave followed by an even larger wave in the fall or winter of this year and one or more smaller subsequent waves next year. This would require lockdowns and social distancing measures to be reinstated in the fall in an attempt to drive down the spread of the virus and prevent healthcare systems from being overwhelmed. This is similar to what occurred in the 1918 Spanish Influenza pandemic and the 1957-58 pandemic.
The third scenario sees the current wave of the pandemic followed by ongoing transmission and case occurrence, but without a clear wave pattern. This has not been seen in past influenza pandemics, but could occur with COVID-19. This would likely not require social distancing measures to be reinstated, but cases and deaths will continue.
The CIDRAP report stressed that no matter which scenario actually occurs, governments must be prepared for at least another 18 to 24 months of “significant COVID-19 activity,” adding that it was likely the virus will eventually fall into a seasonal pattern with diminished severity over time.
The report advised governments to plan for the “worst-case scenario (Scenario 2), including no vaccine availability or herd immunity.” CIDRAP recommended that officials develop “concrete plans” to deal with peaks of the virus, including triggers to reinstate social distancing measures.Lipsitch warned against current efforts by states to lift restrictions. "I think it's an experiment. It's an experiment that likely will cost lives, especially in places that do it without careful controls to try to figure out when to try to slow things down again," Lipsitch told CNN, adding that some of these states have more new infections than when restrictions were first imposed. "It is hard to even understand the rationale."
The CIDRAP report compared the current coronavirus pandemic to past influenza outbreaks, stressing that SARS-CoV-2 cannot be compared to SARS or MERS, as these two viruses behaved in “substantially different” ways than the current coronavirus outbreak and that influenza pandemics are more comparable.
Both SARS-CoV-2 and pandemic influenza virus are viruses to which the global population has little to no pre-existing immunity. They both are predominantly spread through breathing in large droplets, but also by smaller aerosols. Asymptomatic transmission can occur with both viruses as well.
There are a few differences between the novel coronavirus and pandemic influenza virus.
The average incubation period for influenza is about two days. The average incubation for COVID-19 is five days, which means that COVID-19 could move more silently in different populations before being detected.Public health officials currently estimate that 25% of all COVID-19 cases are asymptomatic and this percentage could rise as better serologic studies are conducted. In comparison, the average percentage of asymptomatic influenza cases stands at about 16%, according to one review. SARS-CoV-2 may also be able to spread more before the onset of symptoms than influenza, according to the CIDRAP report.
The report explained that one way to quantify the transmissibility of the coronavirus is to use the basic reproductive number (R0), meaning the number of new infections that result from a single infected person in a wholly susceptible population. The R0 can vary depending on a number of factors, including social distancing measures, lockdowns, behavior and contact with others. An R0 below 1 means that an outbreak is shrinking instead of expanding, as each infected person is infecting less than 1 other person.
Immunity achieved by natural infection or vaccination can influence the effective reproductive number (RE), which is similar to the R0 but doesn’t depend on having a fully susceptible population. Immunity can diminish or end an outbreak by bringing the RE below 1.
The R0 can change from person to person, depending on natural variability in viral shedding and the behavior and contacts of a person, among other factors. There may also be “super-spreaders” of SARS-CoV-2, as there were in the MERS and SARS outbreaks. The R0 at the beginning of the outbreak in China was estimated at 2.0 to 2.5.
The R0 for pandemic influenza has varied but have consistently been estimated around or below 2, which may suggest that even past severe influenza pandemic viruses have been less transmissible than the novel coronavirus, according to the CIDRAP report. Even the highest median RO associated with the Spanish Influenza pandemic was only about 1.8, lower than that of the novel coronavirus.
Reuters contributed to this report.