Mass vaccination leading to herd immunity is our best chance

In a recent cross-sectional serosurvey conducted by icddr,b, antibodies against SARS-CoV-2 (widely known as coronavirus) from blood samples were found in 71 percent and 55 percent of Dhaka and Chattogram city-dwellers respectively (The Daily Star, June 23). More than two-thirds of slum-dwellers developed seropositivity (antibody positive) against the virus, the report added. It means they were exposed to and infected with the virus but did not necessarily develop symptomatic disease. In fact, study results showed that only 35.5 percent of seropositive people had mild symptoms while the rest were asymptomatic.

Antibodies are protective proteins made by the immune system to fight infection, and utilised to assess seroprevalence of an infectious disease of public health importance in a community. In some instances, it is also used for diagnostic purposes in case of diagnostic dilemmas in severe Covid-19 cases, where RT-PCR tests are negative—such as post-Covid multisystem inflammatory syndrome in children—and when antibody detection remains the only laboratory diagnosis. More importantly, during the current pandemic, serological testing appears to be a useful strategy to determine human-to-human transmission of the virus. The utilisation of serological tests for the asymptomatic population or those showing mild symptoms is crucial as they rarely opt for PCR testing, masking the population's actual rate of Covid-19 infection.

However, there is another significant takeaway as well as some fallacies surrounding the seroprevalence of Covid-19. About 72 percent of the population in slum areas were exposed and infected with the virus, the above report said, despite popular belief that Covid-19 has mostly infected people of upper socioeconomic classes so far, sparing the poor. This belief or illusion, however, is not completely shattered from the interpretation of the study results. Although slum-dwellers are more exposed to and infected with the virus than non-slum-dwellers, they rarely developed Covid-19 symptoms. People may be infected with a virus but may not necessarily develop the disease. Subclinical infections producing protective antibodies without causing symptomatic disease are like a blessing in disguise, as it protects the person from future infection through previously formed immunity—although antibodies formed by previous infection may not last long or may not show the same efficacy with different variants of the virus.

High seroprevalence was similarly found in Dharavi, Mumbai, the largest slum in South Asia. The serosurvey carried out by the Brihanmumbai Municipal Corporation (BMC) in July 2020 found 57 percent of slum-dwellers to have developed antibodies against the virus. They were mostly asymptomatic with a very low case fatality rate (0.05-0.1 percent), which resembled the Bangladeshi scenario. High density of population, lack of social distancing, and common utilities like shared toilets could be attributable to high seroprevalence, the BMC report stated, and the low symptomatic Covid-19 caseload in the population was due to effective containment measures. In the icddr,b report, however, no such explanation was given.

People who performed good hygiene practices like washing hands frequently and not touching their nose and face were found in the icddr,b serosurvey to be at a lower risk of exposure to Covid-19 as evidenced by lower seroprevalence. However, people with low exposure to an infectious disease with low seropositivity are more vulnerable to future symptomatic infection from the same virus as they developed no protection from previous subclinical exposure. A published study in Bangladesh, where I was a co-investigator, found that urban adolescents and children of higher socioeconomic status with lower seropositivity against the Hepatitis A virus were more vulnerable to developing severe symptomatic Hepatitis A than their rural and urban counterparts of lower socioeconomic status, who developed higher seropositivity due to previous subclinical infection, as a result of poor hygiene and sanitation practices in their households.

Considering the high prevalence of coronavirus antibodies in the non-representative sample size, a question arises: are we heading towards developing herd immunity? It is one of the most talked-about weapons in fighting a virus. Herd immunity occurs when a virus can no longer spread easily because enough people are already immune to it due to natural infection or from mass vaccination, thereby preventing the virus from jumping from person to person and reaching those who have not yet been infected or vaccinated. For herd immunity to develop, 60-90 percent of the population need to develop immunity through mass immunisation or natural infection. The icddr,b researchers, despite finding reasonably high seroprevalence in their study, ruled out such a possibility, the non-representative sample size being an important reason.

Herd immunity through natural infection, however, can be disastrous even in high-income countries with an enormous loss of lives as the health system will be overwhelmed with patients. It also requires a number of waves of Covid-19 infection to develop, which is why herd immunity is generally pursued through mass vaccination. Immunity (seropositivity) developed from natural infection is also unreliable as there is still poor understanding as to how long it will last. Most importantly, the constantly mutating coronavirus is giving rise to deadlier and more contagious variants which may show resistance to immunity developed from past infection.

India is a glaring example of why relying on natural infection-induced herd immunity may not work. A recent deadly second wave of Covid-19 caused a public health catastrophe there, leading to apocalyptic scenes of overwhelmed hospitals including in the capital. A serosurvey carried out by the Indian National Centre of Disease Control on 21,387 Delhi city-dwellers in June-July 2020 found that 23 percent of residents had developed Covid-19 antibodies with the remaining 77 percent still susceptible. Thus they interpreted that one-fourth of Delhi residents had developed immunity without vaccine or symptomatic Covid-19. Dr Jayaprakash Muliyil, chairman of the scientific advisory committee of the National Institute of Epidemiology in India, mentioned in an interview with the Outlook magazine on July 22, 2020 that judging from the seroprevalence survey, India was "fast approaching herd immunity" and Covid-19 was "going to end on its own when we reach herd immunity".

However, Delhi and other states witnessed a devastating surge of Covid-19 in April and May 2021, where up to 400,000 daily cases were recorded. Clearly, the previous seropositivity didn't help. The emergence of the highly transmissible and immunity-evading Delta variant of Covid-19, together with religious festivals, election rallies, and ignoring early warnings, were responsible for this catastrophe.

Therefore, it is desirable to achieve herd immunity with mass vaccination covering at least 70 percent of the population which can be done faster and without people getting sick or dying. Until that is achieved, we have to protect ourselves by simple health safety measures like putting on a face mask, frequent hand washing, social distancing, maintaining respiratory hygiene, and staying away from social gatherings and the sick.

Prof Dr Md. Salim Shakur is a senior consultant at the Department of Paediatrics, United Hospital Limited.