Covid-19 Herd Immunity Will Be Elusive

The extent to which Covid-19 vaccines protect people from getting infected or spreading the virus to others is still unclear.

Many policymakers, politicians and the media are still harping on herd immunity, as if it was the holy grail in the management of Covid-19. Some have even projected that herd immunity will be the way out of the pandemic. But is this focus based on science? 

Herd immunity (also termed ”population immunity”), is the indirect protection from an infectious disease that occurs when a population becomes immune either through vaccination or immunity developed following previous infection.

Exposing a population, or even a segment of it, to a virus is unethical and scientifically problematic. If the SARS-CoV-2 virus, which causes Covid-19, is allowed to spread through populations, it will lead to unnecessary infections, suffering and death.

The percentage of immune people to achieve herd immunity varies with different infectious diseases. For example, herd immunity in measles is achieved when about 95 per cent of a population is vaccinated. The remaining 5 per cent will be protected because measles will not spread among the vaccinated. On the other hand, the threshold for polio is about 80 per cent. 

The percentage of the population that must be vaccinated against Covid-19 to achieve herd immunity is currently not known.

When a substantial percentage of a population is vaccinated against Covid-19, there will be a reduction in the overall amount of the virus to spread in the whole population. This will protect vulnerable groups who cannot get vaccinated e.g. those allergic to vaccines.

The global scientific community and many governments increasingly believe that Covid-19 will become endemic and are making preparations for it. The reasons for this view are below.

Will Vaccines Prevent Transmission?

The crucial factor in herd immunity is that there would be too few susceptible people who can continue to spread the virus from an infected person i.e. those who had been infected or had been fully vaccinated cannot contract and spread the virus. 

The Covid-19 vaccines are effective at decreasing the likelihoods of symptomatic disease, severe disease and hospitalisation. There is evidence that high vaccination rates reduce the spread of Covid-19. 

However, the extent to which the vaccines protect people from getting infected or spreading the virus to others is still unclear. The resurgence of Covid-19 in countries which have high vaccination rates e.g. Israel raises questions. This is problematic for herd immunity.

The ability of any vaccine to prevent spread need not be 100 per cent. However, if it is less, then there could be substantial viral spread in the community, which would hinder the stopping of viral spread. 

Uneven Vaccine Rollouts And Uptakes

The speed and distribution of vaccine rollouts and uptakes matter. The ideal of a perfectly coordinated global campaign, which, although technically feasible, is impossible in the real world. There are huge variations in the efficiency of vaccine rollouts between and within countries. 

Because of the large numbers of reported cases in the Klang Valley in the second quarter of 2021, concerted efforts were made to vaccinate large numbers of the population. While this was commendable, vaccination rates in other adjacent states lagged behind, some considerably so. The reasons, which are varied, include access to vaccines and vaccine hesitancy. 

Although most infected children have mild or no symptoms, many have been hospitalized and some have died. Children with underlying conditions are more likely to get severe Covid-19, but even healthy children can be similarly affected. Children can spread the infection to other children and adults. 

The Statistics Department estimate that 7.5 million (23 per cent) of the 32.7 million population in 2021 are aged below 14 years. 

Children have been enrolled in clinical trials of some vaccines, some as young as three years old. Although these vaccines will be available soon, and assuming that a decision is made to vaccinate children, it will take time to vaccinate the children. In the meantime, many more adults would need to be vaccinated.

As such, the view that there will be herd immunity just because 80% of adults have been fully vaccinated is fallacious. Even with 100% adult vaccination, this only comprises about three quarters of the population. 

The geographical aspects of herd immunity have to be taken into account. No community is an island unto itself. Covid-19 has occurred because of people’s behaviour and/or local implementation of national policies, which have been, to say the least, a series of flip-flops.

It is important to learn from the resurgence of measles and polio in Malaysia, which have been attributed to non-vaccination of migrant children and localised resistance from anti-vaxxers. 

Notwithstanding the high Covid-19 vaccination rates in the Klang Valley, if the rates in the surrounding states are not the same and populations mix, the potential for new outbreaks remains.    

Impact Of Variants

The longer it takes to stop or even slow down the spread of the virus, the more time variants will have to emerge and spread. 

There are lessons from Manaus, Brazil which was severely affected by Covid-19, with more than 60 per cent of its population estimated to have been infected by June 2020. The decrease in the cases between May and October 2020 were attributed to herd immunity.

But in January 2021, there was a huge resurgence in cases after the emergence of the Gamma (P.1) variant. This suggested that previous infections did not confer broad protection to the virus.

Although it could be questioned whether the 60 per cent figure might have been an overestimate, the fact remains that there was resurgence in the presence of a high level of immunity.

The Delta variant is now the predominant variant in many countries, including Malaysia. The United States’ Centers for Disease Control (CDC) has estimated that its Ro, i.e. estimated number of secondary cases of infections that an infected individual spreads to a susceptible population, is 5-8. 

This variant been held responsible for the surges of large numbers of cases in many countries, even in those with high vaccination rates. 

It is comforting that the current vaccines used are protective against severe disease, hospitalisation and deaths in people infected by the Delta variant. However, the vaccines’ effectiveness is less.

Population data from the United Kingdom showed that the fully vaccinated were three times less likely than the unvaccinated to test positive for the virus. The study also suggested that fully vaccinated may be less likely than the unvaccinated to spread the virus on to others.

The Lambda variant, which spread is more than that of the Delta variant, is a worry for many particularly when the data on the variant is limited.

Higher rates of population immunity can lead to selective pressure on the virus which would favour the development of variants able to infect the vaccinated.

Rapid and widespread vaccination can prevent this development. However, the challenges from uneven vaccine roll-outs and uptakes have yet to be surmounted in Malaysia.    

Immunity Might Not Last Forever

Individuals infected by SARS-CoV-2 appear to have immunity against the virus but how long it lasts remains an unanswered question. Although it is neither zero nor 100, it appears that such immunity wanes with time.

The jury is still out on how long vaccine-induced immunity lasts. 

As vaccines are not 100 per cent effective, an increasing number of countries are providing boosters, some to their health care professionals and the immunocompromised, and others to the general population. 

Time will tell whether boosters are needed. 

The implementation of boosters will have implications for vaccine production and delivery as well as accentuate uneven vaccine roll-outs.

Vaccines And Human Behaviour

As more people get fully vaccinated, it is very probable that there will be an increase in their interactions. This will impact herd immunity as it is dependent on the number of people exposed to the virus.

For example, if an individual interacts with one or two persons prior to vaccination, but interacts with 10 to 15 persons after getting fully vaccinated, would it be getting back to square one?

Unprecedented times and unprecedented human behaviour poses challenges in factoring human behaviour in any modelling.

Non-pharmaceutical interventions (“NPIs”) like physical distancing, face masking, and frequent handwashing have a vital and continuing role in keeping the spread of variants and the numbers down. 

How many fully vaccinated individuals will continue to comply with the 3Cs, i.e. avoid crowded places, confine spaces and close conversations and the 3Ws, i.e. wash, wear and warn?

The examples of some politicians, who influence many, are not encouraging. Will their followers and the public ease off their protective behaviours which would facilitate viral spread?  

Insufficient Testing

It took more than 18 months for the Ministry of Health to disclose that the estimated actual prevalence of Covid-19 is three times that of every reported positive case. 

The disclosure revealed a fundamental flaw in the management of Covid-19 in Malaysia, i.e. insufficient testing, and by extension, insufficient contact tracing and isolation of positive cases. The undetected infected individuals would continue to spread the infection in the community. 

Selective ramping up of testing would go some way in addressing this problem, which impacts herd immunity. 

Being Realistic

The Covid-19 situation is nothing to shout about. Malaysia is in dire straits. 

The number of positive cases exceeded 1.5 million on August 20. It took 482, 65 and 27 days to exceed 500,000, one million and 1.5 million cases respectively.

n the global league of cumulative reported cases, Malaysia was in the 25th position on August 18, up from 39th on May 31 and 85th on 18 November 2020. 

As of August 20, the number of daily cases and daily deaths per million population exceeded that of India, Indonesia, and the Philippines the only Asian countries with more cases than Malaysia.  

The vaccination rate is often used by policymakers and the media as the herd immunity threshold. It should not be considered as a “we are ‘safe’ threshold” but as a “we are ‘safer’ threshold”,

Even after the threshold has been exceeded, outbreaks will still occur — hopefully, these will be isolated. 

It is time to be more realistic in everyone’s expectations. 

The new normal has to be preventing severe disease, hospitalisation and death, as well as limiting viral spread. While Covid-19 vaccines have contributed much to these, it is unlikely that it will completely halt viral spread. 

As such, the emphasis has to be on how Malaysians can live with the virus. Covid-19 may not disappear, but the focus has to be on efforts to make its prominence wane. 

In a nutshell, herd immunity will be elusive.

Dr Milton Lum is a Past President of the Federation of Private Medical Associations, Malaysia and the Malaysian Medical Association. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.

You may also like