By Grant Jacobs 13/05/2021

Many people seem to be reading that our COVID-19 vaccine won’t prevent the spread of COVID-19 from one person to the next.1 If you’re worried about this, this one is for you.

We now have evidence the vaccine reduces transmission, and a rough idea of by how much.2

Science is often about pulling different pieces of evidence together, in this case two large studies from the UK.

The ONS study shows one dose of vaccine reduces the chance of getting an infection from someone else to about one-third (1/3) of that of unvaccinated people. (The vaccine also reduces the severity of those infections. Very few fully vaccinated people need hospital care.)

The PHE study shows vaccinated people who become infected were about one-half (1/2) less likely to infect others in their household compared to unvaccinated people.

Taken together, vaccinated people are about one-sixth (1/6) less likely to infect each other than unvaccinated people are. (1/3 × 1/2 = 1/6)

We’ll do a lot better at facing COVID-19 if everyone who can get vaccinated does.

A slightly geekier summary can be found in the Appendix at the end. This research is yet to be peer-reviewed.

That’s the key story—we now know the vaccine substantially reduces transmission between people.

Moving on

Don’t feel conflicted if you see older communications from the Ministry of Health or Unite Against COVID-19 or similar say that there is no evidence of reducing transmission.

It’s easy: the new advice replaces the old. In science you move on as new evidence comes in. No good comes of being stuck in the mud!

If you see people claiming the vaccine does not prevent transmission, share this post with them.

While we’re here let’s have a quick look at a few other things about the vaccine below. I’ve covered these lightly. If it’s useful to readers, I can address any of these in more detail later.3

I’ve left misleading claims for another time. Some of the other Sciblogs writers listed at the end of the More information on NZ’s vaccine section have tackled many of these earlier.

Why we need to vaccinate almost everyone

At some time we have to open our borders to travel from overseas travel without quarantine. When we do, some people coming in will have COVID-19.

We want to keep transmission to thin chains of cases we can easily track, trace and isolate.

Even with many people vaccinated, there still will be a few people passing it on, just (much) less often. It’s likely this transmission will typically be of smaller viral loads, causing less serious infections.

Note we’re not expecting there to be no transmission from person to person. What we’d like is that what transmission there is doesn’t go far, and is easy to close off.

We need everyone who is able to vaccinate to get their shot.

A small number of people have medical reasons they cannot be vaccinated. Their safety depends on us vaccinating so that any chains of infections are unlikely to get to them.

Possible road bumps

One problem is if the virus gets into groups of people who as a group don’t vaccinate, then it’ll spread within that group.

We also need to be careful that large outbreaks of COVID-19 don’t create new variants our vaccine immunity is less able to defend against. It’s one reason it’s a good idea to help neighbouring countries, and to help squash big outbreaks overseas. As long as there are many cases overseas, they create opportunities for the disease to come to us.

Until we’re past large-scale vaccination we have to be prepared to shutdown quickly and effectively when community cases happen. We also have to keep up with the public health measures. Wear masks on public transport. Keep scanning those QR codes – turn the Bluetooth option on! The scanning needs to be done before community cases happen. It helps our tracing teams shut down any spread quickly. It’s a tiny bit of effort that lets us ‘play hard’.

How effective is the vaccine?

We’ve known for a while the vaccine prompts immunity that strongly reduces symptoms. It’s one of the primary end points of the clinical trial of the vaccine completed late last year. Since then well over a hundred million people have been vaccinated with this vaccine. We know a lot about the safety and effectiveness of the vaccine from monitoring those people.

The clinical trial shows an efficacy of about 95% — compared to unvaccinated people, vaccinated people have 95% fewer cases of COVID-19. If we expect 1,000 infected people (cases) in a group of unvaccinated people, the same number of vaccinated people will have about 50 cases.4 (Most cases in the vaccinated people will be milder.) For the clinical trial, the vaccine had a 100% prevention of hospitalisations or death. Outside of the clinical trial a tiny number of vaccinated people end up in hospital.

It’s one of the most effective vaccines developed.5

Reducing serious cases makes a big difference to health systems and hospitals. One problem with COVID-19 is it’s remarkable ability to overwhelm health systems. India is the obvious example right now. Large scale vaccination prevents this. The UK, USA and Israel have all brought their cases down using large-scale vaccination, along with public health measures.

You have to wait for immunity to develop, it’s not instant! Don’t rush off thinking you are immediately immune. Data from the PHE study indicates it takes two weeks for strong protection to develop.

Communications from the vaccine company and regulatory agencies suggest that full approval for the BioNTech/Pfizer vaccine in the USA is not far off. If this happens, MedSafe will likely come to similar conclusions.

Vaccinating the young

Like most countries, we started by vaccinating older people. This reduces most of the people at risk of being serious cases if there is a COVID-19 outbreak. It also reduces the risk of hospitals becoming overwhelmed if we get another outbreak.

Vaccinating adolescents protects them, and increases community immunity.

Young adults get infected and can carry as much virus as adults.

They less often get seriously ill than adults, but that’s not zero. They can get long COVID, get seriously ill, or even die. They’re not bulletproof.

Don’t think it doesn’t matter because of our lack of cases at the moment.

The idea is to protect them if there is an outbreak, and set them up for when we open up the country. If they were somewhere where there is an outbreak, would you be wishing they were protected?

Before we open the border to open travel, we need to prevent spread through the country. The more people we vaccinate, the more likely we can prevent spread when infected cases come into New Zealand. Until we vaccinate younger people, they remain a source of transmission.

And vaccinating younger people isn’t just about them: it’s also about the people they might infect (and who those who get infected might infect and so on). They can pass their infection on to people who are at a lot more risk that them, including people who cannot vaccinate for medical reasons.

Helping other nations

We can’t (easily) have fully open international travel until everyone, everywhere is vaccinated. Other countries with big outbreaks will be a source of infection to us. If we can help them protect them, we protect ourselves too.

I prefer that New Zealand vaccinate down to some age that covers those at increased personal risk, then help our South Pacific neighbours alongside (or even before) completing our vaccination to younger ages.

You might think of it as creating a South Pacific ‘green zone’.

There’s also a need to try help countries with big outbreaks. Even leaving the humanitarian issues aside, they’re still a problem for us. They affect travel and trade. The large outbreaks can also be breeding ground for new variants that we can’t easily handle.

New Zealand is a small country; our ability to help large countries is limited. But we can help our tiny island neighbours, and perhaps some smaller countries further afield.

Effectiveness against variants

Variants are a big topic that I will try cover in other posts. For now the good news: the vaccine we use in NZ offers protection against all the current ‘variants of concern’. There are slight reductions in effectiveness against some of these variants, but not enough to make the vaccine useless or pointless.

There will be some so-called ‘breakthrough’ cases. Don’t panic because media report one or two happening.

(They are because immunity from vaccines greatly reduces the chance of infection, but doesn’t completely eliminate it. We expect that.)

More information on NZ’s vaccine

There is a lot of information about the vaccine online. A few sources are listed below. These tend cover the more common questions. Feel free to let me know if there is a topic or question you’d like help with – there are several of us that can cover them, or find others to help.

MoH: COVID-19: Vaccine resources (PDF files of brochures in a variety of languages).

Office of the Prime Minister’s Chief Science Advisor: COVID-19 vaccines

Health Navigator New Zealand: Common questions about the COVID-19 vaccine rollout

The Immunisation Advisory Centre: COVID-19 vaccines

More about the COVID-19 vaccines can be found on Sciblogs in Diplomatic Immunity, Infectious Thoughts and bioBlog. Guest posts on a wider range of COVID-19 related topics can be found in Public Health Expert and COVID-19. I’ve written on a few aspects of COVID-19. (I hope I haven’t left anyone out!)

There is a lot of nonsense about the vaccine online. You’re better to use one of the places I’ve linked above. If it’s useful I can cover some of the ideas being shared – let me know.

About the author

I’m a scientist (a computational molecular biologist) and writer. I’ve tracked this pandemic from early on by fortuitous accident. Most of 2019 I was researching zoonoses for writing projects. Part of that was looking for examples of outbreaks. In early December 2019, a ‘pneumonia of unknown cause’ from China caught my attention, and I’ve followed the science and specialists’ discussions closely since. Viruses are molecular life at its minimal, and fascinating for anyone whose work examines how molecules interact and make life happen.

If you want to ask questions, you’re welcome. You’re best to try via Twitter.3

Article by Grant Jacobs, ©2021-.

Other articles in Code for life

Other articles in Code for life


1. The Ministry of Health (MoH) was saying we don’t know if the vaccine prevents transmission in their Facebook replies until very recently. (It’s a reason for this post.) COVID-19 science very moves fast, it’s hard to keep up with it. Some of us on the cutting edge of science communication are quick! The earlier advice from MoH reflects the clinical trials. The clinical trials used to test safety and efficacy of the vaccines are not designed to test reduction in transmission from person to person. To see if there is a reduction in transmission scientists look at what happens when the vaccines are put to use—like the studies I’ve written about here.

The claim vaccines don’t prevent transmission is being made in a pamphlet going around, echoing similar claims on social media. They’re wrong.

2. Scientists anticipated (hoped!) the vaccine would reduce transmission. For example, once you know fewer vaccinated people get COVID-19 (the clinical trials show this) you can anticipate the vaccine likely reduces transmission by some extent, but you can’t easily or reliably say by how much. It’s complicated by transmission before infected people show symptoms, and spread from infected people who never go on to show full symptoms.

Clinical trials cannot test for transmission because they test individuals, not groups in a setting. That’s fine for testing efficacy and safety, which are about individuals.

Some ideas about transmission can be gleaned from detailed case studies of individual settings. They’re fascinating, detective-like puzzles, but the results are hard to generalise.

3. I’m more likely to see your question if you use Twitter.

4. For more, you could try Olliaro, P., What does 95% COVID-19 vaccine efficacy really mean? or Nowogrodzki A, COVID-19 vaccines: What does 95% efficacy actually mean? or other explanations online.

5. It’s a story for another day, but the rabies vaccine series is highly effective – essentially 100% when done properly.

Appendix: preventing transmission with a few more numbers

Several countries with many COVID-19 cases have run large vaccination campaigns over the last few months, including the UK, USA, Israel and elsewhere. Using statistics from these we can test if their vaccination campaigns are reducing spread from person to person.

ONS study

The UK-based Oxford-ONS (Oxford University with the UK Office for National Statistics) survey tracked the outcomes of nearly 370,000 people. Comparing households with a mixture of vaccinated and unvaccinated people they were able to estimate how effective the vaccine was at preventing transmission. Within a household, people would be very likely to become infected if another person was infected.

The family setting used is important. Families are a mostly ‘closed’ setting that you can measure what is happening within them, regardless of whatever else is happening in the wider scheme of things. Using families with a mixture of vaccinated and unvaccinated lets them explore transmission and protection.

They found one shot of the vaccine reduced the odds of infection by about two-thirds (65%, 95% CI 60 to 70%; P<0.001) three weeks after vaccination. Two shots of the BioNTech/Pfizer vaccine gave a slightly bigger reduction, about 70% (95% CI 62 to 77%; P<0.001). (That’s the vaccine we’re using in New Zealand.)

They found both vaccines had a strong protective effect against the B.1.1.7 variant first identified in Kent/UK.

For more, you can read the preprint.

PHE study

Public Health England (PHE) studied, “over 57,000 contacts from 24,000 households in which there was a lab-confirmed case that had received a vaccination, compared with nearly 1 million contacts of unvaccinated cases.”

They have reported that their research shows, “those who do become infected 3 weeks after receiving one dose of the Pfizer-BioNTech or AstraZeneca vaccine were between 38% and 49% less likely to pass the virus on to their household contacts than those who were unvaccinated.”

Note that’s with only one dose of the vaccine. We would expect the effect to be slightly stronger for two doses.

They also found the results were similar regardless of the age of the index case (the infected person) and the contact.

The protective effect was sharply reduced if there were 14 or fewer days since the index case was vaccinated. This will reflect the time it takes for immunity to develop since vaccination.

The B.1.1.7 variant (‘UK/Kent’) became more dominant over the time of the study. There was also a national lockdown that reduced cases and deaths.

The full study can be found in this PDF file.

Featured image

A nurse preparing a COVID-19 patient for a procedure in intensive care. Public domain, source WikiMedia.

Description: “Lieutenant (junior grade) Natasha McClinton, a surgical nurse, prepares a patient for a procedure in the intensive care unit aboard the U.S. the hospital ship USNS Comfort.”

Artist: US Navy Mass Communication Specialist 2nd Class Sara Eshleman.