How effective will the coronavirus vaccine be at reducing the spread of Covid-19?

Three different vaccines have now been approved for use in the UK, but how effective will they be at actually preventing others from being infected with the virus? Olivia Petter investigates

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Last year, when Covid-19 began spreading in the UK, few could have predicted the impact that the virus would eventually have on our daily existence. By May, when the pandemic had already killed thousands, Boris Johnson described the race for a vaccine as “the most urgent shared endeavour of our lifetimes”, explaining that it was the “only” way we could emerge from the pandemic.

But a vaccine for a novel virus, such as Covid-19, that was nowhere near fully understood, would not be easy. Then there was the issue of timing. In the history of medicine, vaccines are very rarely developed in under five years. The mumps vaccine, which was developed in 1967, is one of the fastest-produced vaccinations. It took four years to make.

It was safe to say that the pressure for a coronavirus vaccine was on. And so when, in December last year, the Pfizer/BioNTech vaccine was approved for use in the UK, it was testament to the huge scientific and political will for a solution. The vaccine is the fastest to go from concept to reality in history, having done so in 10 months, when it would ordinarily be expected to take 10 years. Two further vaccines have since been approved: the Oxford/AstraZeneca and the Moderna. This fast tracked approach only made possible by extensive funding and collective interest (each was tested in more than 20,000 people before approval). Finally, it seemed like there was light at the end of the tunnel.

But as England settles into its third lockdown and daily death rates reach an all-time high, there are questions about how effective the vaccine will be at reducing the spread of Covid-19. In December, the UK’s deputy chief medical officer, Professor Jonathan Van-Tam, explained that people must continue to be extremely cautious about transmission even after they’ve been vaccinated. “The magic phrase is transmission,” he said at a Downing Street press conference.

“I think we can be confident and we will know quite quickly within a matter of a couple of months the impact of these vaccines on reducing severe illness in the population, and when we know that we'll be able to say - I hope we'll be able to say - when you're fully vaccinated, your chances of severe illness from Covid are very markedly reduced.”

Although Mr Van-Tam assured the population the vaccine will work to reduce the severity of illness if contracted, he could not make such assurances about transmission. “We don't know if the vaccines will reduce transmission yet,” he said. “I can't give you the assurance that you won't still pose a hazard to others through transmitting the virus.”

Now the vaccine rollout is well underway in the UK, with the government pledging to administer at least two million vaccinations each week, a goal that would see tens of millions of people immunised against the virus by spring and all adults by the autumn. 

But what impact will the vaccine have if people who have been vaccinated can still spread the illness to others? And how will this affect the need for nationwide lockdowns and other restrictions? Here’s everything you need to know.

Will the coronavirus vaccine reduce transmission?

We know from the clinical trials that the coronavirus vaccines are effective in preventing people from becoming unwell - this is because they cause your body to create a rapid immune response. But while all three vaccines offer more than 90 per cent protection against getting ill from the virus, that doesn’t necessarily mean that it stops people from getting infected, or from passing it on. Public Health England literature, given to people after their first dose, says: “The vaccine that you have had has been shown to reduce the chance of you suffering from Covid-19 disease... some people may still get Covid despite having a vaccination, but this should be less severe.”

As Mr Van-Tam said in December, it’s too early to say with any certainty that any of the three coronavirus vaccines will reduce transmission in the population as well as protect against serious illness. PHE says: “We do not yet know whether it will stop you from catching and passing on the virus, but we do expect it to reduce this risk.” This uncertainty around transmission is due to a number of reasons: one indicator is existing PHE data, which shows how people who have been infected with Covid-19 and are carrying antibodies can still transmit a new infection even if they do not appear to be ill.

“It is likely that transmission will be greatly reduced [by the vaccine], but may not be zero,” explains Michael Head, senior research fellow in global health at the University of Southampton. “One of the reasons for this is how the body responds to a naturally-acquired Covid-19 infection. We know that people can spread the virus before they develop symptoms, but also completely asymptomatic cases are likely to be responsible for some transmission too. Therefore, this virus is extremely tricky to properly get on top of.”

Not all vaccines can stop both symptoms and spread

Dr Jenna Macciochi

Immunologist Dr Jenna Macciochi explains this inability to have a dual function (reduce severity of illness and stop transmission) is not rare in vaccines. “Not all vaccines can stop both symptoms and spread,” she says. “Vaccines might either just prevent symptoms but still allow for transmission,” she says. “In an ideal world we would like to have both [this would result in something known as sterilising immunity, whereby a vaccine prevents you from becoming unwell from the virus because you are immune from catching the infection altogether] but it is quite challenging to develop vaccines that do this.”

In fact, the majority of vaccines in routine use today do not stop virus transmission altogether. For example, as Sarah Caddy, immunology and veterinary surgeon at the University of Cambridge, writes in The Conversation, vaccines that combat rotavirus, which causes diarrhoea in infants, are only capable of preventing severe disease. “But this has still proven invaluable in controlling the virus,” she adds, noting how, in the US, there have been almost 90 per cent fewer cases of rotavirus-associated hospital visits since the vaccine was introduced in 2006. “A similar situation occurs with the current poliovirus vaccines, yet there is hope this virus could be eradicated globally,” she says.

Additionally, the influenza vaccine does not necessarily provide complete protection against infection, but it can still provide defence against serious illness and potentially dying from the flu or complications attached to it. “The Covid-19 vaccine is likely to be more like the influenza vaccine in this regard,” says Dr Penny Ward chair of the Faculty of Pharmaceutical Medicine’s Education and Standards Committee. “Because of this, we may need to be revaccinated at intervals (yet to be determined) to maintain protection.”

Dr Macciochi adds that in terms of transmission prospects, vaccinated people would most likely produce far fewer virus particles than those who have a symptomatic infection. “This means that vaccinated people may be less likely to transmit severe disease.”

Why don’t we know whether or not the coronavirus vaccines will reduce transmission?

At this time, there is no official data on the effect that the coronavirus vaccine will have on transmission rates. The reason for this is because the vaccines were not designed for this purpose. The focus was on reducing the number of people who become seriously unwell from the virus. This is fairly standard when it comes to vaccine manufacturing, given, as previously mentioned, how difficult it is to produce a vaccine that can stop both symptoms and spread.

“The clinical trials were broadly speaking designed to answer the question ‘do these vaccines reduce Covid-19 illness in vaccinated populations?’, with particular emphasis on numbers of severe and mild cases,” explains Head. “They weren’t set up to specifically consider whether they reduced onward transmission.”

Additionally, the trials were not designed to evaluate whether the vaccine prevented the spread of the disease among the contacts of vaccinated individuals, adds Dr Ward. “To do that would require the design of other types of trials to determine the risk of infection (which may be without symptoms) in those that have been vaccinated and the frequency of spread of that infection to contacts of those that have been vaccinated.” 

As more and more people are vaccinated - and lockdown is eased, with social interactions increasing, evidence on transmission will be easier to collect on a large scale, in the knowledge that if vaccinated people do contract it they are less likely to be seriously unwell.

Do people that have been vaccinated pose a risk to others?

PHE says it does expect a reduction in risk but cannot give certain assurance that people will not continue to pass it on. As a result, it is still important to “follow the guidance in your local area to protect those around you”, including social distancing, wearing a mask, and washing your hands.

This is still the case even if people are showing no symptoms as we now know one in three people can be asymptomatic. “We can expect restrictions of some kind to be in place for months to come until the vaccination roll-out has covered most of the population,” adds Head.

This is also why those who have been classed as clinically extremely vulnerable have been advised to continue to shield even after they’ve been vaccinated. The government website says that individuals who are clinically extremely vulnerable will have some degree of immunosuppression, or be immunocompromised. “Therefore, those who are clinically extremely vulnerable should continue to follow government advice on reducing their risk of infection,” it adds.

When will we know more?

In order to know for certain how effective the vaccines are at reducing transmission rate, it’s simply a case of gathering the necessary data. “Vaccinated people will need to be tracked over a period of time to determine the population effectiveness,” says Dr Ward. The Department of Health and Social Care has already published their strategy for monitoring how successful the vaccinations are online.

The document states that PHE will be monitoring the effectiveness of the Covid-19 vaccines against several outcomes, including: virologically confirmed symptomatic disease, hospitalisation, mortality, laboratory confirmed infection (symptomatic or asymptomatic), markers of infectiousness and transmissibility, viral load, and onwards person to person transmission.

As more people are vaccinated across the UK, the more data PHE will have to measure the effectiveness of the vaccines and whether or not people who have been vaccinated can still transmit the virus.