There are three approaches to dealing with the Coronavirus.
- Crush the outbreak through testing, isolation and contact tracing
- Try to achieve herd immunity by letting the virus spread while protecting vulnerable populations
- Pursue neither of the above and wait for a vaccine
Crush the Outbreak
The first option is hard. It requires a well coordinated national health response and widespread citizen trust in government. From a look at a few of the countries that have had some success, it helps if your country has a female head of state and your country is an island. Taiwan, Singapore, South Korea, Vietnam, Iceland, New Zealand and Australia are often cited as having done a good job suppressing the virus to very low levels. These countries can safely open up much of their economies because they have isolated all known cases. But of course in a world where the coronavirus is very much out of control, these countries must remain on guard for possible of new infections from outside their borders. Of course if the rest of the world could follow their lead, the risk for all countries could be greatly reduced.
Germany also appears to be pursuing a containment strategy with aggressive testing, isolation and contact tracing. Germany has a female head of state, but is hardly an island. Germany has 9 neighbors with very porous borders.
Though much maligned as a possible strategy for dealing the pandemic it remains a theoretically valid strategy. However, it is dependent on certain assumptions that are still just that: assumptions. The assumption is that having been infected and recovered from COVID-19 confers at least some immunity from reinfection. The degree of protection provided and the duration of that protection remain unknowns. A summary of research on human immune response to the virus certainly indicate some amount of immunity, but it may vary by severity of infection and other factors.
A herd immunity strategy suggests that once a large enough percentage of the population has had the infection, the rate of new infections will naturally decline because of the lack of new susceptible people to infect. The percentage of people who need to become infected and recover is generally estimated to be around 67%. This is based on the assumption that in a population where everyone is susceptible to infection, each infected person will on average infect about 3 other people. This measure is known at R0 (R naught). A second measure referred to the effective reproduction number – R(t) – takes into effect real world factors.
An interesting website (rt.live) provides a real time estimate of the R(t) in each of the US states. In figure 2 below, my state, Massachusetts has an estimate R(t) of 1.01. This means that currently on average, each infected person in Massachusetts will infect 1.01 others during the course of their infection. At this level the number of people will grow at a very, very slow rate. As of the today (7/11/2020) the state with the most rapidly growing infection is Montana with an R(t) of 1.36. It is important not to confuse R(t) with the number of infections in a state. Montana has a relatively small number of new infections compared to states like Florida, Texas and California. But on a percentage growth basis, Montana is growing rapidly.
From the rt.live site we can see the effective reproduction number for US states falls in a fairly narrow range, currently between .85 in Maine and 1.36 in Montana. This is far from the theoretical rate of 3.0. The Journal of Infection published an article outlining what percent of the population needed to have been infected to achieve effective herd immunity in 32 countries. Based on data available in March of this year, they estimated R(t) in the US to be 3.29. That was before extensive lock downs, masks and social distancing. At that level they calculate the US would need 69.6 percent of the population to be infected to achieve herd immunity. However, at that time Korea had an estimated R(t) of 1.43, at that rate, Korea only needed 30.1% of the population to have been infected to achieve effective herd immunity.
The percentage of the population needing to have recovered from infection to reach a point where the outbreak goes into decline, R(t) below 1.0, depends on all the things going on to control the outbreak. Herd immunity can work in conjunction with masks, social distancing and contact tracing to force the pandemic into decline. Based on rt.live data it looks like overall in the US R(t) might be around 1.10. Based on data the Journal of Infection, an R(t) of 1.10 only requires about 11% of the population to be immune for the outbreak to go into decline. Now, the data in rt.live is based on the current situation in each state, and therefor automatically reflects the impact of people in each state who have recovered form the virus. The point is that given the controls we already have in place, the needed increase in immune citizens is nothing like 67%. An increase in 10, 20 or 30% in immune members of the community could easily push R(t) below one and initiate a general decline in infections. It is fair to assume the states hit hard in the early part of the outbreak in the US have had the greatest number of infections (both detected through testing and those that remain undetected). Therefor those states would likely have the greatest percent immune and we would expect to see those states having lower R(t) partially driven by that immunity. In figure 2, the states hit early in outbreak, CT, MA, NY, NJ, MD and DC, are all in the lower half of the rt.live listing.
Sweden is perhaps the poster child the herd immunity. Sweden did not institute tight lockdowns nor did they shutter their economy. Their strategy was to allow the disease to progress naturally in sections of the population that were at the least risk of severe outcomes, while trying to protect those populations at higher risk. Sweden did not close elementary schools, but did close secondary schools and colleges. Early on, Sweden, like many nations, failed to provide effective protection for the elderly, and saw serious spikes in deaths among that population.
Early attempts by Sweden to measure the number of people who have been infected using various antibody tests revealed disappointing results. But its unclear if that was a problem with the tests, which are notoriously inaccurate, or if the infection rate was still quite low. Many articles have pointed out that the death rates in Sweden were much higher than their neighbors who did undertake lockdowns. But in this pandemic, its hard to compare outcomes mid-flight. When one country pursues a strategy that involves letting the infection spread, while another tries to slow infections, it may not be clear whose strategy was most effective until the pandemic is over.
Strategies that seek to allow faster infection among younger and healthier populations while at the same time protecting those at higher risk, and managing the pace of infections to avoid threatening the capacity of the health care system, may be a valid tool in battling the pandemic.
Waiting for a Vaccine
Over the past few months it has been hard to discern a coherent coronavirus strategy in the United States. Initially there was a clear plan to flatten the curve to prevent the health care system from being overwhelmed. Perhaps we are victims of our own success because, aside from a few scary weeks in the New York area, the economic shutdown and social distancing interventions saved the US from replicating the experience of Northern Italy. Since that success we have vacillated between the desire to reduce the spread of the virus and the desire to open up the economy. As I heard one epidemiologist say in a recent interview “…. and we have done neither of them well.”
If we were to seriously pursue a “Crush the Virus” strategy we would need to have a level of testing, isolation and contact tracing far above what we have today. The covidnow website rates state COVID-19 pandemic status on 4 important measures. One of them is whether or not the state has a sufficient number of people working on contact tracing to manage their current level of new infections. Thirty-six states had less then half of the needed tracers. Only a handful of states, mostly in the Northeast had sufficient staff to provide contact tracing for all new infections.
There appears to be very few public health officials willing to discuss herd immunity as a valid alternative. Some of the more enthusiastic supporters of “opening up the economy” have pointed to Sweden as a model that the US should follow. But it appears more based on the belief that COVID-19 is a hoax, rather than a strategy based on sound epidemiological understanding of the virus.
Without a real plan for controlling the virus with existing tools, all eyes and hopes seem to have shifted toward a vaccine, as the only path out of our current chaos. I am sure Dr Fauci has grown weary of answering questions about the plausibility of having a successful vaccine ready to use by the end of the year. Though he says it is possible, he seems to be increasingly hedging on how plausible it is. Prior to COVID-19 the fastest vaccine development process took 4 years. All of the ambiguity regarding the strength and durability of the immunity resulting from infection also sheds doubt on the vaccine process as well. Vaccine’s generally attempt to trigger the development of the same anti-bodies that humans develop in response to an infection by the target disease. If it unclear how effective and durable the naturally occurring anti-bodies are, then the antibodies developed in response to a vaccine are equally suspect.
Having failed to vigorously adopt a plan to control or get through this outbreak, we have come to place all our bets on the rapid development of an effective vaccine. Given the complexities and past track records of vaccine development it seems ill advised to put all our eggs in one basket. At the very least we should start thinking about contingencies, hopefully in conjunction with the international community, regarding plans to crush the virus or to launch a controlled attempt at herd immunity.