The Vaccine Doesn't Necessarily Stop You Spreading Covid-19

HuffPost UK reader Lyn asked: “If I get the vaccine can I still transmit the virus to others?”

Every Monday, we’ll answer your questions on Covid-19 and health in a feature published online. You can submit a question here.

This week, HuffPost UK reader Lyn asked: “If I get the vaccine can I still transmit the virus to others?”

There are many unknowns when it comes to the various vaccines that have been created to fight Covid-19 – and one of those is whether such vaccines can reduce transmission or not. The short answer is: we don’t know for sure yet.

Professor Chris Whitty, England’s chief medical officer, told a press conference in December 2020 that true herd immunity will only occur if we have vaccines that can reduce transmission between people.

At the moment, we know the Pfizer/BioNTech and Oxford/AstraZeneca vaccines can reduce severe illness from the virus, but less is known about their ability to stop people spreading the virus to others.

There are murmurings, however, that such vaccines should have *some* effect on transmission.

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Both the Pfizer and Oxford/AstraZeneca vaccines reduce the incidence of symptomatic infection and so are “likely” to reduce the R value and contribute to an overall reduction in transmission, Professor Paul Hunter, an expert in medicine at University of East Anglia, previously told HuffPost UK. But we don’t know this for certain and further data is needed.

A 2020 press release from Oxford/AstraZeneca states there was an early indication that its vaccine – when given as a half dose, then a full dose – could reduce virus transmission.

This was evidenced by lower rates of asymptomatic infection in those who were vaccinated. It’s generally agreed that the risk of spreading Covid-19 is lower in asymptomatic people than those with symptoms.

A more recent preprint study (meaning it hasn’t been peer-reviewed yet) published in February suggests again that the vaccine could alter transmission. The paper on the vaccine’s efficacy reported a reduction in viral expression after the first dose in vaccine recipients, compared to unvaccinated participants.

Oxford University said the findings of the pre-print paper show the jab has a “substantial effect” on reducing transmission of the disease.

Dr Gillies O’Bryan-Tear, former chair of the Faculty of Pharmaceutical Medicine, says if vaccines reduce transmission to the extent reported, it will mean that the easing of social restrictions could be enabled sooner. “That would be the holy grail of the global vaccine rollout,” he says.

A Pfizer spokesperson told HuffPost UK in December that the matter of immunity after vaccination is a question the company continues to explore in its research – and this will ultimately help them better understand transmission.

“The duration of immunity after Covid-19 requires observing a large number of people who have had the disease once until some get it a second time,” they explained. “Because the first known cases of Covid-19 only occurred in December 2019, there hasn’t been enough time to observe a significant number of second illnesses to know the duration of natural protection.”

Similarly, determining the duration of immunity after vaccination will require observing a large number of people who have been immunised with an effective vaccine to see if or when their rates of Covid-19 will start to increase, relative to the time they were vaccinated.

“We will better understand transmission when we have data on protection for those who were previously exposed to SARS-CoV-2 or infected with Covid-19, asymptomatic disease and severity of the disease,” says the spokesperson.

“Our trial will continue to determine those areas to determine the full protection and potential of the vaccine.”

“The duration of immunity after Covid-19 requires observing a large number of people who have had the disease once until some get it a second time.”

- A Pfizer spokesperson

Kirsten Hokeness, director of Bryant University’s Centre for Health and Behavioural Sciences, in Rhode Island, told Men’s Health there is a “theoretical risk” that you could pass the virus on to others despite being vaccinated. But it’s not a foregone conclusion.

The whole point of the vaccine is to create “immunological memory in the body”, she explained, so that when a person encounters the virus, their immune system quickly works to attack it before it has a chance to make a person sick.

“Therefore, as long as the vaccine boasts a strong immunological response, it is likely that the virus will be stopped from replicating in your system pretty quickly,” she said – and that would hopefully limit your ability to spread it to others. More so than if you weren’t vaccinated, anyway.

For now, the message is: it’s better to be safe than sorry – we don’t know for sure that having the vaccine stops you from spreading it to other people, only that it should prevent you from getting severely ill.

The best thing to do – until enough people are vaccinated, and we have more robust data on how the vaccines impact transmission – is to wear a mask, wash your hands and social distance.

Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit and