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The pandemic has now claimed more than a million lives worldwide, among more than 33.49 million confirmed infections so far. Qatar reports more than 125,000 infections and 214 deaths.
Developing herd immunity is an important way to gain protection against contagious diseases such as smallpox, measles, mumps, polio, and COVID-19. Once achieved, herd immunity can provide some level of protection even to unvaccinated people, mainly due to the significantly reduced possibility of disease spread within the community. There are two ways to develop herd immunity: vaccination and previous infection.
Now a recent study posted on the prepress server medRxiv * September 2020 shows the achievement of herd immunity to SARS-CoV-2 infection by at least some expatriate worker communities in Qatar.
Questions for Researchers
The current study aims to answer whether the chains of infection have actually been broken in any country or community so that transmission can no longer be sustained due to successful herd immunity. And secondly, at what attack rate does herd immunity occur?
Qatar is a small Arabian Gulf country, with a population of 2.78 million, that suffered a strong COVID-19 outbreak. The case rate reached more than 50,000 per million in August 2020, among the highest in the world. So it followed the classic SIR model, which peaked in May, with a steady decline thereafter for the next four months.
Highest infection rate among expatriate workers
The initial community transmission and the highest case rate occurred among the expatriate population, comprised primarily of artisans and manual workers (CMW). This group, which is usually made up of young single men between the ages of 20 and 49, constitutes the majority (60%) of Qatar’s population. In any workplace, these workers work and live in shared rooms, bedrooms and bathrooms, and dining rooms, with food prepared in a common cafeteria-style kitchen.
Most workers remain within their community, limiting infection to discrete community bubbles. Despite the near impossibility of classical social distancing, this allowed the infection to spread rapidly within each community but remain confined there, similar to “flu outbreaks in schools and especially in boarding schools.“
Highlights of the epidemic in Qatar
The researchers draw attention to four characteristics of the outbreak in Qatar:
- The high number of cases per million in CMW
- The high proportion of asymptomatic infection
- The high proportion of positive tests in the random testing campaigns conducted around the peak of the epidemic in various CMW communities
- The classic epidemic curve following the SIR pattern, with a strong decrease in incidence during the four months after the peak, even without strict social distancing
This raises the question “whether herd immunity may have been achieved in at least some of these communities. “The operational definition of herd immunity in this situation was“the proportion of the population ever infected (“attack rate”) beyond which transmission / circulation of infection becomes unsustainable in this population with limited infections, if new ones occur. “To answer this, the researchers conducted serological tests in several CMW communities to detect detectable COVID-19 antibodies.
Among nearly 5,000 CMWs from 10 communities, the researchers found that more than 70% were under the age of 40 and were almost entirely male. Approximately 43% and 33%, respectively, were of Nepalese and Indian origin.
Two-thirds seropositivity rate
Approximately 3,200 seropositive results were obtained from these individual tests, with the highest community rate being ~ 84% and the lowest ~ 55%. On average, about 66% of the tests were positive. When the national COVID-19 database was searched for previous test results on the same individuals, it was found that more than 1,000 had a history of laboratory-confirmed PCR positive infection prior to the current study.
The highest rate of diagnosis ranged from about 28% to 83% in communities that had previously undergone PCR testing due to random testing or as part of contact tracing. In other communities, the diagnosis rate ranged from 0.4% to ~ 13%.
Community disparity in seroprevalence
The highest odds of being positive for COVID-19 antibodies were for Bangladeshis, at almost 7 times the odds for other nationalities, while Nepalese had a ~ 5 times higher risk, Indians and Kenyans ~ 3.5%, while the lowest risk was ~ 3% for Sri Lankans and ~ 2% for Filipinos. The first three were mostly workers, while some Indians, and the last two communities, were mostly managers and administrators, possibly with less contact and different degrees of accommodation than the first.
While women made up only 5% of the group, they were 87% less likely to be HIV-positive. This is probably because they shared accommodation in smaller groups.
With higher education, especially college, the chances of being HIV positive were 75% lower. Symptoms of COVID-19, contact with infected people, or signs of a more serious infection were similar to those not predictive of HIV status.
Low CRP chance
For these CMWs, just over 2000 were analyzed for viral RNA by polymerase chain reaction (PCR). Only 112 were positive, with several communities having a detection rate of zero, while the highest detection rate was ~ 11% in one community. For the combined tests, the PCR positivity was 4%. Interestingly, only 0.8% of seronegative individuals had been diagnosed with PCR positive prior to this.
Furthermore, about 80% of the positive tests came with a cycle threshold value greater than 30, indicating that active infection was not present. There was a notable community-based difference in positivity of infection, either by serological tests or by PCR, from a maximum of ~ 84% to a minimum of 63%. The positivity of the combined test was still ~ 66%.
Only 21 of these individuals had been hospitalized for COVID-19, but only five and one with serious and critical illness. All 21 were positive by serology or PCR. The proportion of people with severe or critical infection of the ~ 3,200 confirmed positive tests was 0.2%.
Cross Herd Immunity Threshold
Overall, therefore, CMW communities had a seroprevalence of 65-70%, which is consistent with the classic estimate for group immunity (at R0 of 2.5-4, this figure matches 1-1 / R0 calculated); a low positive CRP rate, and most of them suggest an earlier infection than an active infection. Furthermore, they did not find a single group of infection during this study, suggesting that transmission is now unsustainable, despite the fact that large groups have been identified quite frequently in these communities during the spike and peak phase of infection. , at the end of May. .
Furthermore, these groups have not been identified over the past weeks in Qatar in any CMW community in the absence of significant social distancing since June 15, 2020. The researchers conclude that their results “support that herd immunity has been achieved (or at least nearly reached) in these CMW communities, and that the level of herd immunity required for SARS-CoV-2 infection is an attack rate (proportion ever infected ) of about 65-70%.“
However, they do not rule out the achievement of group immunity even with a lower attack rate, based on the mathematical model, which indicates that exposure to a new infection in the first round could significantly exceed the level required for group immunity. as above. calculation. This is especially true for a community that allows homogeneous social contact, while a heterogeneous contact rate reduces the number of people infected.
Transcendence
The researchers say this study provides “to our knowledge, the first empirical evidence of herd immunity worldwide. “This process, although limited to specific communities, was achieved in a few months. This could mean that in similar communities, almost two-thirds of the population that must be infected to achieve herd immunity, similar coverage is required for vaccination to achieve herd immunity in a community that begins with zero exposure to SARS-CoV-2.
*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.
