Can we rely on natural herd immunity to end COVID-19 pandemic?

Flu vaccine

All previous influenza pandemics — 1918 Spanish flu, 1957 Asian flu, 1968 Hong Kong influenza, and 2009 H1N1 — came to an end when herd immunity acquired naturally reached a certain threshold. So can India and other countries rely on this to tackle COVID-19 till effective vaccines and anti-virals become available?

With the number of novel coronavirus cases reported so far continuing to increase in most States even during the nationwide lockdown that came into force on March 25, India faces a challenging of containing the virus spread as the May 3 deadline for lifting the nationwide lockdown is fast approaching. With the number of cases increasing from 519 on March 24 to nearly 28,000 on April 27, the lockdown has indeed prevented an explosion of cases.

As the World Health Organization had pointed out on March 25, the unprecedented measures of national shutdown can only buy time and reduce the pressure on the healthcare system. But itself, it cannot “extinguish epidemics”. Contrary to the WHO’s advice of using the opportunity to carry out more precise and targeted measures to chase and contain the virus, there has been no concerted efforts to aggressively test and isolate positive cases even in hotspot and containment areas.

In light of that, will extending the lockdown become a default option as was done after the first phase of restriction ended on April 14 even as the government believes there is no community transmission yet in India. Or as many experts have been advocating, will India bite the bullet and opt for lifting the restrictions in a phased manner?

There are more cases than recorded

Dr. Giridhara Babu, an epidemiologist at the Public Health Foundation of India, Bengaluru and a member of the CoVID-19 task force of Karanataka and Dr. Jayaprakash Muliyil formerly with Christian Medical College say that without doubt there will be an increase in the number of cases once the lockdown is lifted. Particularly so as no effort to hunt the virus was taken during the lockdown period. But a careful, graded approach to lift the restrictions can still help prevent a huge spike in cases and concomitant straining of the healthcare system.

Any strategy adopted for lifting restrictions should bear in mind that the actual number of people who have been infected is many times more than the laboratory-confirmed cases. Dr. Babu estimates that the total number of infected in India to be around 2,50,000 while Dr. Muliyil says that for every recorded case there are 60 people who have not been counted.

Strategies to be adopted

While extending the lockdown in hotspot areas appears not only prudent but also essential, more focus during the remaining period of the lockdown should be on silent areas that have not reported any case or just a few cases. For instance, Sikkim, which has walled itself up, and a few other States have not reported a single case, and many northeastern States have reported only a few cases. These States would not need continued lockdown, once the true infection prevalence is ascertained. Increased surveillance of people exhibiting severe acute respiratory illness (SARI), influenza-like illness (ILI) and any COVID-19 suspect cases in the silent areas will help determine if restrictions should be eased or continued after May 3.

Most importantly, the decision to continue the restrictions or not should be taken at the local level; centralised approach to decision making will be hugely counterproductive.

As Dr. Babu strongly advocates, it is important to segregate essential and non-essential activities and encourage more people to work from home till such time an effective vaccine or anti-viral becomes available. Universal mask wearing, physical distancing and maintaining hand hygiene will help in curtailing the spread but will not be practical in slums and other crowded neighbourhoods.

The question of herd immunity

This brings to the fore a more natural way to slow down and bring the epidemic to an end through herd immunity that happens naturally during an epidemic. Herd immunity arises when a sizeable population gets naturally infected over a period of time so that the virus does easily find a susceptible host to infect thus bringing the epidemic to a halt, which is what Dr. Muliyil and virologist Dr. Jacob John advocate.

The herd immunity that these scientists are referring to is very from the one practised by UK Prime Minister Boris Johnson who intended on using it as a strategy to end the epidemic. While Mr. Johnson put even the elderly and those with comorbidities at risk of infection through his approach, both these scientists have kept the interests of the elderly and those with comorbidities as top priorities. While these two category of people who may experience serious symptoms and even death can protect themselves by self-isolation  — also called as reverse quarantine — the young, who mostly exhibit only mild symptoms, can go out by following containment measures together with universal mask wearing, physical distancing and hand hygiene.

Strictly following containment measures, wearing a mask and maintaining physical distancing slows down the rate of infection but also slows down the rate at which herd immunity is achieved naturally. But that’s a small price to pay in order to avoid overwhelming the healthcare system, if many require hospitalisation.

How earlier influenza epidemics ended

In the case of H1N1, the pandemic died out in two years when 40% of the population where the virus was spreading had been naturally infected. But it comes back each year as the susceptible population through new birth grows beyond the threshold. The same was true for all the previous influenza pandemics — 1918 Spanish flu, 1957 Asian flu, and 1968 Hong Kong influenza. Even during the pre-vaccinations days, measles used to strike once in three years and German measles every seven years. Dr. John expects at least one year for herd immunity to develop naturally for novel coronavirus, provided shutdowns are eased.

Though other strains of coronavirus, which cause common cold, infect people almost every winter, the same strain does not infected people twice. But the mutations that these viruses undergo make most people susceptible to infection. Fortunately, the novel coronavirus appears more stable (experience fewer mutations) than other influenza viruses and so an infected person may probably be protected against future disease, Dr. Muliyil hopes.

But WHO cautions that “there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection”. Infected people may have some level of protection against the virus, but the level and duration of protection is still not known. In South Korea, over 200 people who have recovered from the disease have again tested positive. It is still not clear if it is a case of reinfection or reactivation (where infected people have not been able to completely clear the virus).

Published in The Hindu on April 29, 2020