The False Dilemma: WHY YOU DON′T HAVE TO CHOOSE BETWEEN SAVING LIVES OR SAVING THE ECONOMY. WHY SAVING THE ECONOMY NOW, CAN SAVE LIVES LATER?

We are gradually learning more about this new virus. However when everything started, we received several "dogmas" from authorities and scientists. One of them was that we will achieve herd immunity when SARS COV-2 infects 70% of the population. The human mind, said John Maynard Keynes, is like the egg and the sperm: "once a sperm enters the egg it is impossible for another sperm to enter", so are the ideas; once an idea enters in our mind it′s very difficult to removed it and accept a new one. Although there are times when due to the overwhelming evidence against the original theory, we end up leaving the old ideas and accepting the new ones, which Thomas Kuhn called a paradigm shift. For this physicist, the acquisition of new knowledge is not continuous, but happens intermittently; most of the advances in a subject are generated around an initial theory without contributing anything "really new". When a large amount of arguments against classical knowledge accumulate, the old theory is abandoned in favor of the new one (e.g. from Newton's mechanics to Einstein's theory of relativity).

In the case of SARS-Cov-2 any argument that we listen today is based on two assumptions:

1.- The epidemic will behave like the 1918 Flu and the “Outbreak in October” will be even more virulent.

2.- When the vaccine arrives, or when we reach the herd immunity (70% population infected) we will overcome the situation.

The first point I have already discussed it in previous articles: The comparison of the coronavirus with the H1N1 flu pandemic is equivalent to comparing a Ferrari and a Fiat 600 (2 cars, but very different). Moreover, we don′t know yet if human being is a reservoir of the virus, which would keep the virus from disappearing.

But today I would like to talk about the second point: herd immunity. It′s a very relevant concept because when we reach it, it will allow us to return to normal life. Two articles have been published on this subject that shed some light, and give us an idea of what may happen in the future (Individual variation in susceptibility or exposure to SARS-CoV-2 lowers the herd immunity threshold(1); The disease-induced herd immunity level for Covid-19 is substantially lower than the classical herd immunity level(2))

Before I start explaining them, I would like to clarify 2 terms:

R0: It's the basic reproduction number of the virus: how many people are infected from one person (virus carrier) in a specific period. Scientists say it′s between 2.2 and 3.5 (A virus carrier could infect 2 to 3 of their contacts).

This concept is very important to calculated herd immunity (hC = 1 - 1/R0) The higher the rate of reproduction the higher the percentage of the infected population is needed to reach herd immunity.

But how does herd immunity work? Why 70%? Herd immunity is a concept developed from vaccination, if you get about 70% of the population vaccinated, even though the rest are not vaccinated they will be protected anyway because the virus does not find people to infect and replicate itself. The classical herd immunity level hC is defined as hC = 1 ? 1/R0, where R0 is the basic reproduction number defined as the average number of new infections caused by a typical infected individual during the early stage of an outbreak. This definition originates from vaccination considerations: if a fraction v is vaccinated (with a vaccine giving 100% immunity) and vaccinees are selected uniformly in the community, then the new reproduction number is Rv = (1 ? v)R0. From this it is clear that the critical vaccination coverage vc = 1 ? 1/R0; if at least this fraction is vaccinated, the community has reached herd immunity, as Rv ≤ 1, and no outbreak can take place(2)”

The 70% estimate is based on a homogeneous population. Perhaps you know an example of a family member or a friend who, despite living with an infected patient, has tested negative for COVID19 antibodies. We need to explain an important concept to understand this situation: susceptibility. To be able to get infected we need to be exposed to a high viral load, if we are in contact with many people with a high viral load (e.g. healthcare professional) we will be more susceptible than if we live in isolation in a rural area and have sporadic contact with a COVID 19 patient. Therefore, the concept of susceptibility depends on the area where you live, the population density, whether you use public transport or not; in short, how exposed you are to other potentially infected people.

New studies on herd immunity tell us that it could be reached with about 20% of population infected (the most optimistic projections) or 43% (the most conservative ones). They consider that populations are not homogeneous, but very heterogeneous (number of contacts, urban vs. rural...)

Figure: Minimum herd immunity level required (%hD=heterogeneous group and %hC=classical homogeneous group) for different R0 = 2.0, 2.5 and 3.02

Secondly, we have the concept of vulnerability. It's the likelihood of having a serious illness that can be life-threatening. As we have already seen, elderly people, hypertension, diabetes, obesity... are risk factors that can contribute to a serious and sometimes fatal disease

Measures to reduce social contact such as confinement, sanitary masks, avoidance of large community events etc. are intended to reduce the susceptibility of infection by COVID19, while the vaccine or treatments are intended to protect the vulnerable. Maintaining very restrictive measures does not directly reduce deaths as we are attacking susceptibility and not directly vulnerability. In order to protect the vulnerable people, we need treatments or vaccines.

If we review the two studies that tell us that the level of herd immunity is (in a heterogeneous population) between 20% and 40%, we could relax the confinement measures even more and much faster. In fact, these mathematical models tell us that maintaining measures that are too restrictive for too long, can favor a second outbreak because there is a certain population that due to its characteristics (urban, public transport user, high social interaction...) will be infected in any case (purple line in the graph below). Don't get me wrong, social distancing measures are necessary and have helped us a lot. They are necessary to avoid the collapse of the healthcare system (leaving space to treat vulnerable population) but at some point, they are no longer effective.

Figure: Diagram of the total fraction infected over time according to age and community activity with R0 = 2.5. Four different levels of prevention. Blue, red, yellow and purple curves correspond to no measures, light, moderate and restrictive preventive measures respectively(2)

If you are still reluctant to these arguments, there is an unintended experiment: Diamond Princess cruise. As you may know, the cruise ship was quarantined with 3,700 passengers at the coast of Japan. The virus vanished with 17% of the cruise ship population infected, i.e. herd immunity was achieved with 17%. From that number we can infer that the viral load became so low (due to social distancing measures and the sick people got recovered) that the susceptibility to COVID 19 of the non-infected passengers decreased, and no further infections occurred.

We talked about the theoretical R0 that is between 2 and 3, but in real life is close to 0, Most people do not transmit. That′s why we need to consider not only R0 but also k (dispersion factor) The lower k is, the more transmission comes from a small number of people (disease clusters). The k value for the 1918 pandemic Flu was close to 1 (no disease clusters), 2002 SARS had a k of 0.16 and 2014 MERS had a 0.25 k factor. What about SARS Cov-2? Guess what… it′s far from 1918 pandemic k factor and close to SARS and MERS k value = 0.1 k factor of COVID19 (Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China) “Probably about 10% of cases lead to 80% of the spread” said one of the authors.

That′s why some very early cases (they found cases in Italy and France in December) failed to ignite the outbreak. Therefore, by identifying disease clusters (as South Korea did) we can control the pandemic without damaging the economy by taking drastic (and unnecessary) measures as total confinement.

In summary, the question of protecting economy or health is a false dilemma. You can protect both at the same time, it does not make any sense to prolong unnecessary measures that could even be devastating by causing a hypothetical (and unlikely) 2nd outbreak. “Countries where suppression of the initial outbreak was more successful, such as Austria, have acquired less immunity and therefore the potential for future transmission in the respective populations remains naturally larger. However, also in these situations, expectations for the potential of subsequent waves is much reduced by variation in susceptibility to infection(1).”

Groucho Marx used to say: “Those are my principles, and if you don't like them... well, I have others” It's not a question of changing our principles but it seems that herd immunity is not a magic or an unchangeable number. I can imagine that after reading this article your first reaction won′t be positive. However, you can start your own survey: ask friends and family, who has COVID19 antibodies and who doesn’t, and you will see that the virus is not as we thought and maybe that leaves space for new ideas...