- As COVID-19 immunization continues around the world, many people are still unclear what the vaccines mean for transmission.
- Deborah Fuller, a microbiologist, answers three questions about transmission after vaccination and whether new variants could affect it.
- Vaccination does not prevent you from becoming infected, but it does reduce the chance of catching it or becoming seriously ill.
- If a vaccinated person catches COVID-19, the chance of it being transmitted is lower due to the reduction of the virus load that can be transmitted.
So you got your coronavirus vaccine, waited two weeks for your immune system to respond to the shot and are now fully vaccinated. Does that mean you can go around the world like the old days without spreading the virus? Deborah Fuller is a microbiologist at the University of Washington School of Medicine and works on coronavirus vaccines. She explains what science shows about transmission after vaccination – and whether new variants can change this equation.
1. Does vaccination completely prevent infection?
The short answer is no. You can still become infected after being vaccinated. But your chances of getting seriously ill are almost nil.
Many people think that vaccines work like a shield, blocking a virus from infecting cells completely. But in most cases, someone who is vaccinated is protected from disease, not necessarily infection.
Every person’s immune system is a little different, so if a vaccine is 95% effective, it just means that 95% of the people who receive the vaccine will not get sick. These people can be completely protected from infection, or they can become infected but remain asymptomatic because their immune system eliminates the virus very quickly. The remaining 5% of those vaccinated may become infected and become ill, but are unlikely to be admitted to hospital.
Vaccination does not prevent you 100% from becoming infected, but it does give your immune system a big bone on the coronavirus in all cases. Whatever your outcome – whether you have protection against infection or at a certain level of disease, you will be better off if you came across the virus than if you were not vaccinated.
Vaccines prevent diseases, not infections.
Image: National Institute of Allergy and Infectious Diseases, CC BY
2. Does infection always mean transmission?
Transmission occurs when enough viral particles of an infected person end up in the body of an uninfected person. In theory, anyone infected with the coronavirus could possibly transmit it. But a vaccine will reduce the chance of that happening.
In general, if vaccination does not completely prevent the infection, it will significantly reduce the amount of virus coming out of your nose and mouth, and it will shorten the time you shed. This is a big deal. A person who spills less virus will be less likely to transmit it to someone else.
This seems to be the case with coronavirus vaccines. In a recent pre-print study that has not yet been peer-reviewed, Israeli researchers tested 2,897 vaccinated people for signs of coronavirus infection. Most had no detectable virus, but infected people had a quarter of the amount of viruses in their bodies, as people who were not vaccinated were tested for infection at similar times.
Less coronavirus virus means less chance of spreading it, and if the amount of virus in your body is low enough, the probability of transmitting it can be almost zero. However, researchers do not yet know where the coronavirus is, and since the vaccines are not 100% protected against infections, the Centers for Disease Control and Prevention recommends that people still wear masks and social distances even after being vaccinated.
New, more contagious and transmissible variants of the coronavirus may limit the effectiveness of current vaccines.
Image: AP Photo / John Raoux
3. What about the new coronavirus variants?
New variants of coronavirus have emerged in recent months, and recent studies show that vaccines are less effective against certain ones, such as the B1351 variant, which was first identified in South Africa.
Each time SARS-CoV-2 repeats, it gets new mutations. In recent months, researchers have found new variants that are more infectious – meaning that someone has to inhale less virus to become infected – and other variants that are more transmissible – which means that it increases the amount of virus that someone spills. And researchers, according to early data, also found at least one new variant that seems to be better at evading the immune system.
So, how does this relate to vaccines and transmission?
For the South African variant, vaccines still offer more than 85% protection against serious illness with COVID – 19. But if you count mild and moderate cases, it offers at best only about 50% -60% protection. This means that at least 40% of people vaccinated will still have a strong enough infection – and enough virus in their body – to cause at least moderate illness.
If vaccinated people have more viruses in their bodies and it takes less of the virus to infect another person, the vaccinated person is more likely to transmit these new strains of the coronavirus.
If all goes well, vaccines will reduce the rate of serious illness and death worldwide very quickly. To be sure, any vaccine that reduces the severity of diseases reduces, at the population level, the amount of virus that is generally shed. But due to the emergence of new variants, vaccinated people still have the potential to spread the coronavirus to other people, vaccinated or otherwise. This means that it will probably take much longer before the vaccines reduce transmission and that populations will achieve herd immunity, if these new variants have never emerged. Exactly how long it will take is a balance between how effective vaccines are against emerging strains and how transmissible and contagious these new strains are.