COVID: Why experts say herd immunity is still out of reach

Despite vaccinations, coronavirus cases continue to rise across the globe, from the UK, India and Russia to Malaysia. With experts in Germany now talking of an impending fourth wave, many people want to know once and for all: "When will this end?" 

Since the beginning of the pandemic, the term "herd immunity" has come to symbolize the point at which enough people will be immune to the coronavirus that we can hug each other once again, relieve our overburdened health care workers and say goodbye to COVID-19. 

But what is this hazy holy grail exactly, and why does it seem forever out of reach? 

Herd immunity: explained 

Adam Kleczkowski, a professor of mathematics at the University of Strathclyde in Scotland, likens "herd immunity" to a forest fire running out of dry wood. When there is not enough left to burn, the fire runs out of fuel and disappears. When enough people are resistant to the coronavirus, either through infection or vaccination, the virus cannot spread, and the pandemic stops growing and starts to decline.

The percentage of immune people needed to achieve this level of community resistance centers on the reproductive (R) number. This is the average number of people an infected person will pass a disease on to at a certain point in time. When the R number is below 1, meaning when an infected person is not likely to infect more than one other person, the disease starts to run out of fuel and die off.

"We can achieve this by waiting long enough so most people in society have been infected, or by staying socially distant and in lockdown forever, or by vaccinating enough people," Kleczkowski told DW. "The key thing to understand is that not everybody needs to be immune — there is a point at which enough people are immune that the fire can no longer spread," he said. 

So, what’s the key 'point' for the coronavirus? 

At the beginning of the pandemic, scientists estimated this figure to be around 60 to 70%. But throughout the last year and a half, that goal post has been shifting. Recently, experts said the percentage is closer to 80 or even 90%.  

That's due to several reasons.  

Variants pose a problem 

One issue is that this critical number is dependent on how infectious a virus is — how fast it spreads. For measles, which is highly infectious, this percentage is 95%, but for influenza, it can be as low as 35%.  

At the start of the outbreak, the R number for the coronavirus was estimated to be between 2.5 and 3. But as more infectious variants have emerged, it has become more transmissible. 

The Delta variant, which was first seen in India, is around 64% more infectious than the Alpha (Kent) variant, first identified in Britain, which was already 50% more contagious than the original coronavirus variant that spread from China.  

The faster the virus spreads, the greater immunity it will take to slow the infection rate. “That shifts this number up. We might need as much as 85% immunity to slow the Delta variant," Kleczkowski said.  

But, he points out, percentages such as this are only estimations. "They are based on limited data. It is not completely clear what percentage we need to reach," he said.

Vaccination crucial to herd immunity 

Kaja Abbas, assistant professor of disease modeling at the London School of Hygiene and Tropical Medicine, says vaccination is key to reaching herd immunity.  

"Herd immunity through vaccination is the preferred pathway," he told DW. "Herd immunity through natural infection will come at a colossal loss of human life and suffering," he said.  

A recent study out of Israel shows vaccination has not only been successful in protecting vaccinated individuals from severe courses of COVID-19, but also in significantly reducing transmission of the virus.  

That level of immunity requires that a significant portion of the population is vaccinated, Abbas said — which also means ensuring vaccines are administered evenly across the globe.

"We’re not safe until everyone — everywhere — is safe," Kleczkowski said.  

While the UK and the US are nearing full vaccination of 50% of their populations, and Israel is nearing 60%, just over 3% of people in India and 11% of Brazil’s population have received both vaccination doses — two countries where more than 350,000 people have died from COVID. 

What's more, people may also need to be re-vaccinated with a third or fourth dose to protect against new variants of the virus.  

A new normalcy 

Mathematical herd immunity calculations are also unable to take into account the complexities of human behavior. Once a certain figure of immunity is achieved, it could be the case that control measures like mask-wearing, physical distancing and border closures are relaxed, and as a result, new outbreaks occur and herd immunity is lost.  

For these reasons, Kleczkowski says defining herd immunity using a concrete figure "is not helpful." 

Rather than focusing on whether we reach 70 or 80% immunity in the population, Kleczkowski says it’s more useful to think about herd immunity as a process of stamping out the virus until it is eliminated.

That doesn’t mean we need to let the virus rip through communities, he adds, or live in an endless strict lockdown. Instead, it's important to maintain a combination of control measures, such as ongoing testing and mask-wearing in places where infection rates are high, in tandem with widespread vaccination and re-vaccination in response to new variants. 

Even if the world cannot entirely eradicate the coronavirus — which has only been achieved once before with smallpox —  experts such as Abbas and Kleczkowski say vaccines will largely protect people from the worst effects of COVID-19 if infections continue to break out into the future.