Sweden tried it and failed miserably. Britain almost considered it before opting for a lockdown instead. And as we speak, the idea continues to divide experts on science and ethics both. It's called herd immunity, the latest buzzword in the Covid-19 response space. However complicated the concept may sound, there is actually some easy science behind it. Let's unpack.
Left unchecked, a virus can move freely through the population causing a great number of infections along the way. Depending on the virus and its biology, many of the infections may result in deaths. Many of those infected recover. But something else happens as well.
How immunity works
Before understanding herd immunity, it is essential to get a grip on what immunity is. It’s an extremely confusing piece of science involving dozens of moving parts in hundreds of combinations doing thousands of specialised tasks. We don’t need to learn all of it. Just the fundamentals will do.
Any infection is, in essence, an intrusion. Something that doesn’t belong in the body manages to get in, and starts multiplying and hurting the body. We are slathered in hundreds of millions of bacteria and viruses at all times. But thanks to our first line of defense, the skin, we sit insulated. The problem starts when something breaches this line of defense, say a pet’s claws or a rusty nail. Then the second line of defense swings into action. This involves guard cells known as macrophages.
These gigantic cells, nearly 100 times the size of a coronavirus virion, are voracious gluttons, devouring up to 100 intruders each. In order to make the task easier with improve mobility, they signal nearby blood vessels to release more water at the site of the invasion. This manifests as inflammation.
When macrophages spend too long fighting, the third line of defence kicks in – the neutrophils. They are true killers; they kill indiscriminately, not sparing even healthy cells. That’s why they are programmed to self-destruct after five days.
The final line of defence brings in the brain of the immune system, the dendritic cells. When the neutrophils signal fatigue, the dendritic cells mobilise to pick samples of the enemy and carry them to the nearest lymph nodes. This takes about a day or two. The idea is to manufacture specialised weaponry and mount a final all-bets-off assault.
Lying in wait in the lymph nodes are billions of helper and killer T cells. T cells are biological assassins that go through a training so rigorous upon birth that only one out of four comes out alive. The T cells are tested by the dendritic cells against the samples at hand. Should all go well, the T cells start a process of rapid division. Some of them remain in the lymph nodes as memory T cells, some travel to the site of the infection to participate in the battle, and a third group seeks out virgin B cells from the lymph nodes. These B cells are specialised to produce antibodies. Upon instruction from helper T cells, millions of antibodies are launched into the bloodstream. These then latch on to the intruders’ outer surfaces and beacon the macrophages to spot and devour them. As soon as the war is over, all killer cells commit suicide to prevent further damage to healthy cells, but some stay put. These are memory helper T cells and memory B cells.
Should the same intruder hit the body again, the memory cells can start producing the relevant antibody right away, and save crucial time. This is called immunity.
The only problem with this process? Time.
The whole process from macrophages to the training of T cells to antibodies takes time. Sometimes enough for the invading bacterium or virus to overwhelm the system and win before help could arrive. Memory cells help bypass some of the steps and tilt the outcome to an extent.
But these memory cells do not survive forever. Depending on the disease, the body discards unused memory cells after some time to save resources. This span can range from six months to forever.
This is how one develops immunity against a disease: by contracting a disease first and then developing the right memory cells. Problem is it’s risky. The first infection, for one, could prove fatal. One way to avoid such risk is vaccination. A vaccine is a weakened sample of a disease causing microbe that triggers the dendritic cells into creating the relevant memory cells. This way, should the disease in question actually hit, the right antibodies can immediately be mass-produced. And since memory cells die after a while, vaccines need to be repeated at intervals. We call them booster shots.
In the near term, though, a person who has fought back the disease can prove highly valuable. Plasma from a recovered patient that’s still saturated with the right antibodies can be introduced into another patient’s bloodstream to help the fight against the intruder. This is one of the methods being used now to treat Covid-19.
How herd immunity works
Herd immunity is exactly what it sounds like: immunity accorded to a person by virtue of everyone around them being immune. In a non-immune population, the virus can move freely and rapidly from one host to another. But even if one person happens to be immune to the virus in this chain, the trajectory is paused. This individual gets the virus but puts a stop on further movement. This is called “breaking the chain”.
The more such individuals in a “herd” the more bottlenecks for the contagion. And when this number hits a certain value, the bottlenecks override the spread and the contagion practically vanishes. At that point, even a non-immune individual is relatively safe because everyone around them is immune; there’s simply no source of contagion. This is called “herd immunity”.
So, how many people must be immune for herd immunity to kick in? That depends on how rapidly the virus in question spreads. The rate of the spread is referred to as the R0 value of the virus. It’s basically the number of people an infected person can infect in turn. For the novel coronavirus, this value is 1.22 in India. This means, one coronavirus carrier infects up to 1.22 individuals on an average. This value, of course, changes rapidly when there is an active outbreak.
Once this value is determined, we can calculate the herd immunity threshold as Qc = 1-1/R0
For Covid-19 in India, this value comes to 0.18. This means that 18 percent of Indians, or just over 200 million people, ought to be immune to the virus for herd immunity to kick in. That’s not a small number by any stretch of the imagination.
Herd Immunity to the common cold
Herd immunity is the reason we don’t die of such diseases as measles or smallpox anymore. Polio is practically extinct. But less than a couple of generations ago, these diseases weren’t just commonplace, but rather fatal. Measles was particularly infectious with an R0 of 12.
These diseases seem to be making a comeback, though. That’s for two reasons. One is increasingly aggressive anti-vaccination campaigns. As more and more people shun vaccination, viruses are discovering new pathways through hosts that mount no resistance their vaccinated counterparts could. This puts in particular risk the people who cannot be vaccinated such as certain babies, geriatrics, and pregnant women.
Another reason is mutations. Viruses and bacteria multiply rapidly and the faster they multiply, the quicker they mutate to resist existing antibodies. This is an ongoing battle of evolution which is why new vaccines need to be developed at all times. The quicker a virus multiplies, the sooner its vaccine becomes ineffective.
This is the reason we don’t have an effective vaccine against the common cold. Because the common cold virus, which is also a coronavirus, mutates so rapidly that any existing antibody becomes useless in less than a year. So creating a vaccine is going to be a lot of constant work and a costly affair. And since the common cold isn’t particularly deadly, nobody has bothered to work on making a vaccine.
Herd immunity for Covid-19
From what we know of SARS-CoV-2, the virus causing Covid-19, any vaccine will need upgrades or “boosters” every few months. Maybe a year but not more. As we have explained, given the current rate of spread in India, immunising about 250 million people should accord herd immunity to the whole country.
That, however, isn’t easy simply because no vaccine exists as of now. This leaves us with another slightly problematic alternative: letting the virus move about freely and infect as many people as it does. Some will die, others will recover. Those who recover will contribute to the 250-million mark. Those who die will...well, die.
Also, remember that antibodies from recovered patients can be used to treat those with a compromised immune system. So that is one way to reduce mortality. But there will be some deaths, that’s the bitter truth.
Why embracing the virus isn’t a great idea
In the scenario that we still don’t have a working vaccine, the only alternative we are left with is mass exposure. There are several problems with this approach. The first, of course, is the high mortality. Although this value is ever-changing in an active contagion, for Covid-19 in India, it is now at five percent. This means that five out of every 100 confirmed Covid-19 patients are going to die. To achieve 250 million recoveries, we will need 263 million people to get infected, of whom over 13 million will die.
That is a big moral bottleneck in and of itself. Is it alright to knowingly let that many people die for herd immunity? And that’s a conservative estimate. Given how the situation pans out, the fatality rate could go higher.
Another problem is that of sustainability. Even if we let that many people die and achieve herd immunity, we know that coronaviruses mutate very rapidly. So any antibody immunising us from them will become ineffective in a few months. Then the whole cycle will have to start afresh.
So, what’s the way forward? Frankly, we don’t yet know for sure. Fortunately for us, several vaccines are in the works and nearing completion. Some should be market-ready as soon as the end of this year. That’s when we can start being hopeful. Until that point, the best strategy is to space out the infections.
This spacing-out is necessary for two reasons. First, to avoid overwhelming our healthcare system. Second, to slow down the rate of infection and minimise deaths until the vaccines show up. The only way to achieve that is to limit physical interactions as far as practically possible.
Remember, the ideas of social distancing and breaking the chain are neither myths nor government propaganda to cover for its inefficiencies. This outbreak really is that disarming. And the only way out is by staying out of the virus’ way.
Amit Schandillia is a history, science, and political awareness evangelist. Having worked in the American subprime lending space, he now writes on wide-ranging issues with a focus on debunking misinformation.