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A recent study posted on the prepress server medRxiv* in October 2020 suggests that the case for herd immunity is weakened by the fact that the sharp increase in the number of deaths from COVID-19 as a result of being exposed to the virus many times rather than one or a few times times it is not taken.
While most have mild COVID-19, a severe illness associated with diffuse pneumonia may require hospitalization. Many in this group will eventually require intensive care, with supplemental oxygen and mechanical ventilation.
In a considerable minority of cases, death occurs due to multi-organ failure. Survivors in this group may experience permanent disabling symptoms.
The blocking strategies were designed to buy time for productive and preparatory measures by slowing the rate of viral transmission. Vaccines are intended to neutralize immunity to the virus and prevent endemicity. Since it is an RNA virus, it is destined to undergo numerous mutations, which will make it difficult to eradicate endemically.
Another school of thought holds that herd immunity is the best way to handle the pandemic. Without any intervention, the population will develop immunity naturally, while the economy will remain stable, according to the argument. However, the head of the World Health Organization (WHO) has called this an unethical idea.
Problems with herd immunity
Researchers for the current study say herd immunity appears attractive only because it is supported by simplistic simulations that ignore the challenges of real-time healthcare. Furthermore, long-lasting immunity to SARS-CoV-2 has not yet been demonstrated.
Again, this model does not account for the higher death rates that will occur with an overburdened healthcare system as infections rise. And finally, it ignores the long-term disabling effects of this disease, the medical and social costs of caring for such individuals.
Uncontrolled viral spread will cause endemic COVID-19. New variants may emerge that are more destructive to lung tissue. Worse still, multiple exposures can lead to a higher viral load and thus a more significant proportion of severe disease.
Repeated exposures could also trigger an antibody-dependent enhancement (ADE) in the severity of the disease. This is one area where wearing face masks in public all the time could make a huge difference to the burden placed on hospitals and healthcare workers by preventing serious infections.
There are many different variants of the virus in circulation at the present time. These variants show different degrees of infectivity and pathogenicity. It is unclear if these can exist simultaneously in a patient and how they interact, if so.
Underestimation of mortality from baseline data
Since current mortality estimates rely on information from the early part of the pandemic, they are likely to be inaccurate as, at this time, it was unlikely that people would have repeated contacts with infected people. An exception was healthcare workers, as their daily and repeated contacts with many potentially infected people made them capable of superinfections.
Understand multiple infections
The present study aims to understand how mortality is related to the presence of multiple variants of SARS-CoV-2 acquired through multiple exposures. This model also examines how measures to reduce contact with infected people affect superinfections and mortality. It uses a model adapted from the free CovidSIM tool.
The researchers assumed stages of disease from latent (3.7 days), prodromal (1 day), fully contagious (7 days), and late infectious (7 days), and also assumed a doubling of infectivity in the fully contagious stage in compared to the first two. . They found that multiple infections had a ~ 64% risk of causing severe symptoms and 4% mortality compared to 58% and 3%, respectively, of single infections.
Morbidity from multiple infections
Multiple infections caused more symptomatic infections, thus more isolation, resulting in less spread and a smaller peak. But while the total number of multiple infections drops somewhat, especially during the peak of the epidemic, the number of deaths increases.
However, seasonal infections affect this relationship, peaking higher and narrower if it overlaps with the start of the flu season. In this case, there will be more multiple infections and many more deaths than expected.
Severity of symptoms
The severity of symptoms is measured by the number of infected people who seek medical attention and go into isolation, with a direct relationship between the two parameters. Further isolation of multiple symptomatic infections reduces the total number of infections somewhat, reducing the peak of the epidemic and multiple infections.
However, total mortality increases due to the higher number of deaths caused by multiple infections.
Increased mortality
Again, multiple infections reduce the total number of cases, but increase the number of deaths in proportion to the fatality rate of multiple infections. Again, this is compounded by seasonality, with a higher epidemic peak, more multiple infections, and more deaths if the peak occurs early in the flu season.
Risk of spreading multiple infections
Multiple infections can be contracted from one exposure or consecutive exposures. If it is the latter, the effects will be delayed and, in this period, the spread is limited, while recovery can occur rather than death. Therefore, this subset of infected people does not significantly increase the total number of multiple infections.
However, the higher the risk of spreading multiple infections, the higher the epidemic peak and the higher the fatality.
Susceptibility to successive infections.
Previous infection with a SARS-CoV-2 strain can confer partial immunity to multiple infections, depending on the stage of infection of the contagious person. Susceptibility is significantly lower and therefore the risk of multiple infections if the susceptible person is in the late infectious phase.
Although variations in susceptibility do not affect the epidemic peak, they do affect the number of cases of multiple infections. Around the peak, multiple infections are more likely from exposure to two or more single infections than from exposure to a single multiple infection.
Contact reduction and a second lock
The researchers also found that reducing inter-individual contacts in a variety of settings ranging from a second block to no intervention would result in a delayed but no less epidemic peak in a non-seasonal setting. Isolating the case reduces the peak. In both cases, multiple infections and deaths are not affected.
With seasonal changes, fewer contacts can delay the peak of the epidemic to coincide with that of the flu season, increasing the height of the peak. The narrow and sharp peak slightly limits the number of cases of multiple infection and the number of deaths is also somewhat lower.
A second blockade, properly timed, can reduce the number of infections that occur after the relaxation of the first, if the epidemic reaches its peak while the reproduction number is decreasing. The earlier that lockdown is put into effect, the more it will overlap the initial peak of the pandemic and the longer the delay will be to the final peak. This will also result in a wider but flatter peak.
The number of cases and deaths can be further reduced by extending an early closure. However, if it is too late, the lock will not be effective as cases are already decreasing.
Transcendence
The researchers note, “Multiple infectious contacts and longer (average) exposure to the virus are initially restricted to certain risk groups, but will become common during the peak of the pandemic. “
Different variants of SARS-CoV-2 can be acquired at different points by exposure to different infectious individuals. Viral diversity will also only increase during a pandemic.
Herd immunity becomes a less attractive concept given the higher viral load and risk of multiple infections as the pandemic progresses, along with more significant morbidity and mortality. Herd immunity becomes a less attractive concept than a second block at the right time. Results may vary with duration of immunity and human behavior. For example, some people may choose to avoid most forms of contact with other people. The model parameters can be modified for several of these differences.
The researchers also note, “In particular, multiple infections are not the only danger when seeking herd immunity. An uncontrolled (or poorly controlled) pandemic inevitably makes the virus endemic. “
If so, it will not only be difficult to get rid of the virus, but it could find reservoir hosts in domestic animals, escaping control measures. As viral diversity increases, it could also increase in virulence, requiring vaccines to adapt each year. Multiple rounds of vaccination will then be required to eradicate the virus, resulting in ADE in some individuals.
The study concludes “Increased morbidity and mortality due to multiple infections is an important but supervised risk, particularly in the context of herd immunity. Evidence-based research on multiple infections is needed. “
*Important news
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.
