The pandemic has now claimed over a million lives worldwide, among over 33.49 million confirmed infections so far. Qatar reports over 125,000 infections and 214 deaths.
The development of herd immunity is an important way of gaining protection from contagious diseases, such as smallpox, measles, mumps, polio, and COVID-19. Once achieved, herd immunity can provide some level of protection even to non-vaccinated individuals primarily because of the significantly reduced chance of disease spreading within the community. There are two ways to develop herd immunity: vaccination and previous infection.
Now, a recent study published on the preprint server medRxiv* in September 2020 shows the achievement of herd immunity for SARS-CoV-2 infection by at least some expatriate worker communities in Qatar.
Questions for the Researchers
The current study aims to answer whether infection chains have truly been broken in any country or community such that transmission can no longer be maintained because of successful herd immunity. And secondly, at what attack rate does herd immunity occur?
Qatar is a small country in the Arabian Gulf, with a population of 2.78 million, which suffered a heavy COVID-19 outbreak. The case rate reached over 50,000 per million by August 2020, among the highest in the world. It thus followed the classical SIR model, having peaked in May, with a steady decline after that over the next four months.
Highest Infection Rate Among Expat Workers
The initial community transmission and highest case rate were among the expatriate population, mostly comprising craft and manual workers (CMWs). Typically consisting of single young men between 20-49 years, this group makes up a majority (60%) of the Qatar population. At any given workplace, these workers work and live together with shared rooms, dormitories and bathrooms, and dining rooms, with food prepared in a cafeteria-style common kitchen.
Most workers stay within their community, thus restricting the infection to discrete community bubbles. Despite the near-impossibility of classic social distancing, this allowed the infection to spread rapidly within each community but to remain confined there, similar to “influenza outbreaks in schools and especially boarding schools.”
Salient Features of the Epidemic in Qatar
The researchers draw attention to four features of the outbreak in Qatar:
- The high case number per million in CMWs
- The high proportion of asymptomatic infection
- The high positive test proportion in random testing campaigns carried out around the peak of the epidemic in various communities of CMWs
- The classical epidemic curve following the SIR pattern, with a sharp decrease in incidence over four months following the peak even without stringent social distancing
This gives rise to the question “whether herd immunity may have been reached in at least some of these communities.” The operational definition for herd immunity in this situation was “the proportion of the population ever infected (“attack rate”) beyond which infection transmission/circulation becomes unsustainable in this population with limited if any new infections occurring.” To answer this, the researchers carried out serologic testing in a number of CMW communities for detectable COVID-19 antibodies.
Among almost 5,000 CMWs from 10 communities, the researchers found that over 70% were below 40 years, and almost entirely male. About 43% and 33%, respectively, were Nepalese and Indian in origin.
Two-Third Seropositivity Rate
About 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 tests were positive. When the national COVID-19 database was searched for prior test results in the same individuals, it was found that fully over 1,000 had a history of laboratory-confirmed PCR-positive infection before the current study.
The highest rate of diagnosis ranged from 28% to ~83% in communities that had been broadly tested by PCR earlier due to random testing drives or as part of contact tracing. In other communities, the rate of diagnosis varied from 0.4% to~13%.
Community-Wise Disparity in Seroprevalence
The highest odds of being positive for COVID-19 antibodies were for Bangladeshis, at almost 7 times the odds of other nationalities, while Nepalis had a ~5-fold 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, with possibly less contact and different grades of accommodation than the former.
While women comprised only 5% of the group, they were 87% less likely to be seropositive. This is probably because they shared accommodations in smaller groups.
With higher education, especially university education, the odds of being seropositive were 75% lower. Symptoms of COVID-19, contact with infected people, or signs of more severe infection were similar to non-predictive of seropositive status.
Low PCR Positivity
For these CMWs, just over 2,000 were tested 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 pooled testing, the PCR positivity was 4%. Interestingly, only 0.8% of seronegative individuals had been diagnosed as PCR-positive before this.
Even more, about 80% of the positive tests came with a cycle threshold value above 30, indicating that active infection was not present. There was a striking community-based difference in infection positivity, by either serologic or PCR testing, from a high of ~84% to a low of 63%. The pooled test positivity was still ~66%.
Only 21 of these individuals had been hospitalized for COVID-19, but only five and one with severe and critical disease. All 21 tested positive by serology or PCR. The proportion of people with severe or critical infection out of the ~3,200 confirmed test-positives was 0.2%.
Herd Immunity Threshold Crossed
Overall, therefore, the CMW communities had a seroprevalence of 65-70%, which agrees with the classical calculation for herd immunity (at R0 of 2.5-4, this figure agrees with the calculated 1-1/R0); a low PCR positive rate, with most of these being suggestive of earlier infection rather than active infection. Moreover, they did not find a single cluster of infection during this study, suggesting that transmission is now unsustainable, despite the fact that large clusters had been identified quite commonly in these communities during the rising and peak phase of infection, at the end of May.
Moreover, such clusters have not been identified for the last few weeks in Qatar in any CMW community in the absence of meaningful social distancing since June 15, 2020. The researchers conclude that their results “support that herd immunity has been reached (or at least nearly reached) in these CMW communities, and that the level of herd immunity needed for SARS-CoV-2 infection is an attack rate (proportion ever infected) of about 65-70%.”
However, they do not rule out the attainment of herd immunity even at a lower attack rate, based on mathematical modeling, which indicates that the exposure to a novel infection in the first round could significantly overshoot the level required for herd immunity as per the earlier calculation. This is especially so for a community which allows homogeneous social contact, while a heterogeneous contact rate brings down the number of people ever infected.
The researchers say this study provides “to our knowledge the first empirical evidence for herd immunity worldwide.” This process, albeit limited to specific communities, was achieved within a few months. This could mean that in similar communities, just about two-thirds of the population required to be infected to achieve herd immunity, similar coverage is required for vaccination to accomplish herd immunity in a community starting with zero exposure to SARS-CoV-2.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.