IS HERD IMMUNITY AN ILLUSION?

 

As Covid-19 vaccination rates pick up around the world, people have reasonably begun to ask: how much longer will this pandemic last? It is an issue surrounded with uncertainties. But the once-popular idea that enough people will eventually gain immunity to Covid-19 to block most transmission - a ‘herd-immunity threshold’ - is starting to look unlikely.

Long-term prospects for the pandemic probably include Covid-19 becoming an endemic disease, much like influenza. But in the near term, some scientists are contemplating a new normal that does not include herd immunity. 

Here are some of the reasons behind this mindset:-

It is unclear whether vaccines prevent transmission.

The key to herd immunity is that, even if a person becomes infected, there are too few susceptible hosts around to maintain transmission - those who have been vaccinated or have already had the infection cannot contract and spread the virus. The Covid-19 vaccines are capable of preventing symptomatic disease, but it is still unclear whether they protect people from becoming infected, or from spreading the virus to others. These uncertainties pose a problem for herd immunity.

Herd immunity is only relevant if we have a transmission-blocking vaccine. If we do not, then the only way to get herd immunity in the population is to give everyone the vaccine. Vaccine effectiveness in halting transmission needs to be “pretty darn high” for herd immunity to matter, and at the moment, the data are not conclusive. How well the vaccines stop people from transmitting the virus will have big implications. A vaccine’s ability to block transmission does not need to be 100% to make a difference. Even 70% effectiveness would be “amazing”. Unfortunately, there is still a substantial amount of virus spread that would make it a lot harder to break transmission chains.

Vaccine roll-out is uneven

The speed and distribution of the vaccine roll-outs matters for many reasons. A perfectly coordinated global campaign could have wiped out Covid-19 at least theoretically. It is a technically feasible thing, but in reality, it is very unlikely that we will achieve that on a global scale. The reason is because of the huge variations in the efficiency of vaccine roll-outs between countries.

In most countries, vaccine distribution is stratified by age, with priority given to older people, who are at the highest risk of dying from Covid-19.  Everything is now pointing to the need for almost every adult to be immunized in order to achieve herd immunity.  

Another important thing to consider, is the geographical structure of herd immunity. No community is an island, and the landscape of immunity that surrounds a community really matters. Covid-19 has occurred in clusters in every country as a result of people’s behaviour or local policies. Previous vaccination efforts against other diseases suggest that the uptake will tend to cluster geographically too. Localized resistance to the measles vaccination, for example, had resulted in small pockets of disease resurgence. Geographic clustering is going to make the path to herd immunity a lot less of a straight line, and essentially means we will be playing a game of whack-a-mole with future Covid-19 outbreaks. Even for a country with a high vaccination rate, if the surrounding countries have not done the same and the populations are able to mix, the potential for new Covid-19 outbreaks remain.

New variants change the herd-immunity equation.

Even as vaccine roll-out plans face distribution and allocation hurdles, new variants of Covid-19 are sprouting up that might be more transmissible and resistant to vaccines. We are now in a race with the new variants. The longer it takes to stem transmission of the virus, the more time these variants have to emerge and spread.

There is another problem to contend with as immunity grows in a population. Higher rates of immunity can create selective pressure, which would favour variants that are able to infect people who have been immunized. Vaccinating quickly and thoroughly can prevent a new variant from gaining a foothold. But again, the unevenness of vaccine roll-outs creates a challenge. We have got a fair bit of immunity, but we still have a fair bit of disease, and we are stuck in the middle. Vaccines will almost inevitably create new evolutionary pressures that produce variants, which is a good reason for the authorities to build the infrastructure and processes to monitor for them.

Immunity might not last forever.

Calculations for herd immunity consider two sources of individual immunity - vaccines and natural infection. People who have been infected with Covid-19 seem to develop immunity to the virus, but how long that last remains a big question. Given what is known about other coronaviruses and the preliminary evidence for Covid-19, it seems that infection-associated immunity wanes over time, so that needs to be factored in to calculations. However, we are still lacking conclusive data on the waning immunity.

The modellers would not be able to count everybody who has been infected when calculating how close a population has come to the herd-immunity threshold. And they will have to account for the fact that the vaccines are not 100% effective. If infection-based immunity lasts only for something like a few months, that provides a tight deadline for delivering vaccines. It will also be important to understand how long vaccine-based immunity lasts, and whether boosters are necessary over time. 

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