Parts of Brazil may be close to reaching herd immunity

. Aug 31, 2020
Parts of Brazil may be close to reaching herd immunity Photo: Lightspring/Shutterstock

“Herd immunity” has been one of these terms that the pandemic brought into our daily lives. In broad terms, it is achieved when the number of survivors with immunity against a given disease increases to a certain level, slowing down — and eventually stopping — the spread of the virus in question. It was believed that this stage would be reached for Covid-19 if 60 percent of a given population developed antibodies.

Some governments have tried to apply the herd immunity strategy, such as in Sweden or a famously short-lived attempt in the United Kingdom. But these moves, highly controversial as they are, have not proven successful. 

A paper published by the Journal of the Royal Society of Medicine states that Sweden’s higher rates of viral infection, hospitalization, and mortality compared with neighboring countries may have serious implications for Scandinavia and beyond. And in the United Kingdom, Prime Minister Boris Johnson eventually U-turned from his initial coronavirus approach toward herd immunity.

Now, mathematical models suggest that coronavirus herd immunity could be reached after just 10 to 20 percent of people develop antibodies, depending on the specific characteristics of a place and its population. But that doesn’t mean herd immunity should be a goal public authorities should seek. At a news briefing last week, World Health Organization officials called pursuing such a strategy “very dangerous.”

Rodrigo Corder, an infectious disease modeling researcher, explained the fundamental principles of his studies to The Brazilian Report. As a Ph.D. candidate at the University of São Paulo’s Institute of Biomedical Sciences, he is part of a group of bio-mathematicians and health researchers trying to break down and understand the Covid-19 spread and establish the herd-immunity threshold for different areas.

</p> <p><strong>Why do you believe that the threshold for &#8220;herd immunity,&#8221; or collective immunity, may be lower than initially considered?</strong></p> <blockquote class="wp-block-quote"><p>Mathematical models usually consider populations to be homogeneous. We assume that the populations are heterogeneous, contain different susceptibilities and different exposures to the coronavirus. Some have a higher risk, and others have a lower risk of becoming infected. At the beginning of the infection, those with the highest risk are expected to get infected first, leaving only those at low risk for subsequent infections. With those at higher risk already infected, the virus would find it more difficult to circulate, moving towards a threshold of collective immunity.</p><p>Reaching the threshold of collective immunity does not mean the end of transmission or the end of the pandemic. It just means that the virus will not have the same strength to spread and that the number of cases tends to drop even after the relaxation of the control measures.</p><p>The homogeneous models described the beginning of the transmission well, but as they consider all people to have the same risk, they began to have difficulties after a particular moment. Considering this risk distribution, our models continued to describe the incidence data even after the number of cases began to drop considerably.</p></blockquote> <p><strong>What defines vulnerability?</strong></p> <blockquote class="wp-block-quote"><p>There are two characteristics: susceptibility and exposure. Susceptibility is related to immunological factors, contact with other viruses in the past — which may give some partial immunity — or genetic factors.</p><p>Exposure is related to connectivity. A person with many friends is more likely to infect and be infected than someone who has contact with fewer people. Exposure also helps to explain why different locations have different thresholds of collective immunity. There are cultural factors, people that get closer physically, the infrastructure of cities, transportation. In Brazil, in Latin America, there is a culture of getting closer or touching, more than in the countries of northern Europe, for example.</p></blockquote> <p><strong>What is known about the immunity threshold?</strong></p> <blockquote class="wp-block-quote"><p>We know from other diseases that this threshold varies within a certain range. For the coronavirus, we reach between 10 and 20 percent of the population — depending on each population&#8217;s particularities. With the development of the pandemic, we were able to verify, with a certain standard of accuracy and based on serological studies, this threshold for four countries. Belgium was 9.6%, England 20%, Spain 12% and Portugal 7.6%.</p><p>But it is important to remember that mathematical models are approximations of a given event and can never be interpreted as an exact truth. These are values ​​to be compared with other estimates — the homogeneous models estimated between 50 and 60 percent. It is possible to find places outside of the range. Setting an exact value would be irresponsible for people working in modeling because authorities could use it to relax isolation before collective immunity is achieved.</p></blockquote> <p><strong>What is the threshold for Brazil?</strong></p> <blockquote class="wp-block-quote"><p>We do not have the final numbers for Brazil yet, but they will not be far from other countries, between 10 and 20 percent. But Brazil has different dynamics. You can&#8217;t think of Brazil as something unique like we did for Belgium.</p><p>There is also the fact that the <a href="">states</a> had autonomy in managing the pandemic, in distancing measures. It would be much more consistent to analyze each state. And even within states, there are different dynamics. We know that <a href="">Covid-19 arrived mainly in the state capitals</a>, mostly in those with connections with abroad, such as São Paulo, Rio de Janeiro, Manaus, and Fortaleza.</p><p>Transmission starts in the capital and only then migrates to smaller cities, so today we see capitals decreasing, and the interior does not — the curve seems stable. It is expected that exposure will be different between big cities and the interior [of states], there is not much public transport there, the threshold tends to be lower. But we are still analyzing the data.</p></blockquote> <p><strong>What is the current stage of the pandemic in Brazil?</strong></p> <blockquote class="wp-block-quote"><p>It is hard to say that the worst is behind us when around 1,000 people still die every day. But it is expected that the number of deaths will not increase compared to what we have already seen. The moment is still extremely critical; control measures remain necessary. But, as I said, there are several different stages.</p><p>The isolation level in Rio de Janeiro today is low and even so, the number of cases has not increased exponentially as the homogeneous models predicted. This may be evidence that collective immunity is close to being achieved.</p><p>On the other hand, in Brazil&#8217;s interior, the Center-West region took longer to have problems with the disease. So it appears to be at an earlier stage of the pandemic.</p></blockquote> <p><strong>Is Brazil close to collective immunity?</strong></p> <blockquote class="wp-block-quote"><p>The number of people infected by each infected person in Brazil is below 1 for the first time. But this is the general number; there are locations where this number is much smaller than 1 and others where it is much larger and the curve is still growing. It is positive news, but it needs to be looked at carefully.</p><p>A second wave is unlikely in communities where collective immunity has been achieved. But there are different realities, even within the same city. Studies are showing that São Paulo, for example, already has an estimated 18 percent of its adults infected. Still, peripheral neighborhoods have a higher prevalence of antibodies precisely because they were most affected at the beginning of the pandemic. A second wave is possible in places where the incidence of Covid-19 was lowest.</p></blockquote> <p><strong>Is the reduction of Brazil&#8217;s cases the result of public policy or just the natural behavior of the disease?</strong></p> <blockquote class="wp-block-quote"><p>Generalizing is a little unfair, as there were a few politicians committed to control. But in general, we have seen complete neglect, a failure to manage the pandemic.</p><p>In a few places, managers were close to scientists and listening to implement what was needed. But in a continental country, it is difficult for one locale, alone, to succeed surrounded by other locations that have not taken action. Municipalities or states are not islands. If this had happened — with the federal government and the Health Ministry&#8217;s support — we would undoubtedly have had better management.</p></blockquote> <p><strong>President Jair Bolsonaro said Covid-19 is like rain, that everyone will get wet. Is relying on herd immunity as public policy a justifiable solution?</strong></p> <blockquote class="wp-block-quote"><p>Collective immunity is not <a href="">public policy</a>; relying on it is a catastrophic error. Communities that begin to ease control measures before reaching the threshold will have a higher number of cases. That&#8217;s what’s called the final epidemic size.</p><p>Roughly speaking, it can be said that the final value in places where there is little intervention in the pandemic will be about twice the threshold of collective immunity. If the collective immunity threshold is around 20 percent, about 40 percent of the population will likely be infected at the end of the epidemic when no control measures are taken. Taking the necessary actions — with a slow and gradual easing, observing the numbers — the epidemic’s final value should be very close to the threshold of collective immunity.</p><p>When immunity is adopted as a goal, many more people will become infected; consequently, more people will die. And it is possible to avoid this with control measures.

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José Roberto Castro

José Roberto covers politics and economics and is finishing a Master's Degree in Media and Globalization. Previously, he worked at Nexo Jornal and O Estado de S. Paulo.

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