Saturday 27 February 2021

Mutating coronavirus: reaching herd immunity just got harder, but there is still hope

A year after the first cases of COVID-19 were found in China, mass vaccination programmes are now offering a chance to end the pandemic. At the same time, new strains, some of which appear to be more infectious than the original one, are threatening to derail progress in fighting the disease.

One way to capture the effectiveness of our effort to fight the virus is to consider the concept of “herd immunity”. During an epidemic, people become infected and those who survive usually become immune. Resistance to the disease can also be reached by vaccination, which complements the levels of natural immunity. Infected people increasingly come into contact with immune people rather than those who are still susceptible. Like a forest fire that runs out of dry wood, at the point of reaching herd immunity, the epidemic stops growing and starts to decline.

- Advertisement -

Mathematical models help us relate herd immunity levels to how fast the disease spreads. The percentage of immune people needed to create herd immunity depends on the reproductive number, R, which is a measure of how many people each infected person will infect on average. We can reach herd immunity either by waiting long enough so that large parts of the society pass through infection, by reducing the value of R using social distancing forever, or by vaccinating enough people.

Letting the epidemic run unchecked threatens to overload the health system and will result in many deaths. Conversely, lockdowns are causing economic, social and mental health issues. So we hope vaccines will stop the epidemic more effectively.

But how many people will need to be vaccinated to stop the disease spreading? This depends on how infectious the viral strain is, how effective the vaccine is, and how much we are prepared to continue with lockdown measures. The model predictions for the success of the vaccination programmes depend on the vaccine being able to limit the transmission of the virus – not just stop people getting ill. But we still don’t know if the current batch of approved COVID vaccines stops transmission.

Scenarios for vaccine coverage
Critical vaccination levels depend on the estimated effective reproductive number, with the vaccine efficiency of 90%.
Adam Kleczkowski

We could continue with very strict social distancing indefinitely, bringing R below 1 so that the number of new infections decreases. In this case, we would not need any vaccination (point marked A on the graph). But the moment control measures were relaxed, R would jump back above 1 and the epidemic would start to spread again.

- Advertisement -

Or we could combine some level of lockdown with the roll out of vaccines. In this scenario (point marked B on the graph) we would see a decline in cases once 40% of the population were immune to the virus. However, this strategy relies on keeping R just above 1. The disease would probably come back as soon as further relaxation occurred.

As we want to return to our activities and freedoms without social distancing, the vaccination levels need to be much higher, taking into account how fast the virus can spread with no additional control.

A reliable estimate of the rate of spread of the first variant of SARS-CoV-2, during the first stages of the epidemic, is R=3. In other words, every person infected with the coronavirus passes it on to three other people, on average. The corresponding herd immunity level for this value of R is 67%, so assuming a vaccine efficacy of 90%, we need to vaccinate 74% of the population (point marked C) to stop the virus from advancing further.

How does the emergence of the new variants change this value? We do not know for sure how fast the new strains would have spread if allowed to do so without any mitigation measures. The evidence suggests that the UK strain is 30-50%, and maybe as much as 70%, more infectious than the original one. Choosing the value of 50%, the needed vaccination levels are pushed up to 86% (point marked D).

An even higher proportion will need to be immunised if we want to push the virus towards eradication or to guard against a potential underestimation of the rate of spread.

Can such high levels of protection be attained at all? If a vaccine is only 50-70% effective, as the Oxford and Sinovac vaccines appear to be, this is the highest herd immunity level that can be achieved, if everybody gets the vaccine. Even with the higher efficacy of the Pfizer and Moderna vaccines, vaccinating nearly 90% of the population will be very difficult, as not everybody can or will want to be treated.

- Advertisement -

On the positive side, a previous COVID infection still provides good protection against reinfections and so we can probably count on nearly 20% of the population having natural immunity.

The herd immunity levels depend on R values and we can, to some extent, manipulate them by lockdowns. If countries can keep R reduced to just above 1, eradication can be achieved even with smaller vaccination levels. But what is really needed is a coherent long-term plan: one that includes both quick vaccination roll out and continuing social distancing and testing.The Conversation


This article, Adam Kleczkowski, Professor of Mathematics and Statistics, University of Strathclyde, is republished from The Conversation under a Creative Commons license. Read the original article.

- Advertisement -

We strive for accuracy in its reports. But if you see something that doesn’t look right, send us an email. The Q reviews and updates its content regularly to ensure it’s accuracy.

Q Costa Rica
Reports by QCR staff

Related Articles

In a week, Costa Rica off and on the list of risk countries in Spain for covid-19

QCOSTARICA - Spain had excluded Costa Rica from the list of...

97 labs test those who need to leave the country for covid-19 antigens

QCOSTARICA - Need to leave the country and require a test...


Banana growers achieved record exports in the year of the pandemic

QCOSTARICA - Costa Rica's banana growers reached an export record in 2020, despite the impact of the pandemic, shipping fruit in new markets and...

The Multiplaza Escazu store that sells nothing but helps in a big way

QCOSTARICA - Dona Rosa lost everything in her life and surrendered her will to the hands of others. Luck changed one day when she...

The “marías” resist dying in a technological shakeup for taxi drivers

QCOSTARICA - The disappearance of the taximeters (popularly known as "marías") will not be immediate despite the new mobile application that will manage services...

Passengers will be able to make purchases from their cell phone in stores at SJO

QCOSTARICA - Passengers passing through the Juan Santamaría International Airport, or San Jose airport (SJO), will have a more agile and secure shopping option...

‘Bye’ to the weekend license plate restrictions

QCOSTARICA - The nighttime vehicle restrictions starting on March 1 will be from 11:00 pm and not 10:00 pm as this month, and on...

ESPH charges the family for electricity for January and February even though fire destroyed their home in December

QCOSTARICA - Despite losing their home entirely in December, a Heredia received electricity bills from the Heredia Public Service Company (ESPH) for January and...

Can you pay your bills by playing baccarat online?

Casino and baccarat is a term of love. Gambling is known to bring a lot of happiness provided your luck supports you, but how...

There is no money to start the Costa Rican Space Agency

QCOSTARICA - Costa Rica President Carlos Alvarado assured that the State lacks the money to start the Costa Rican Space Agency (Agencia Espacial Costarricense...

False dentist worked from his garage and charged ¢ 100,000 for ‘braces’ that come off in a few hours

QCOSTARICA - A 30-year-old man, who posed as a dentist, set up an office in the garage of a house in Siquirres, Limón, where...


Get our daily newsletter with the latest posts directly in your mailbox. Click on the subscribe and fill out the form. It's that simple!

Log In

Forgot password?

Forgot password?

Enter your account data and we will send you a link to reset your password.

Your password reset link appears to be invalid or expired.

Log in

Privacy Policy

Add to Collection

No Collections

Here you'll find all collections you've created before.