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What COVID-19 vaccines can and can't do

Louisa Wright
February 8, 2021

From virus transmission to infection, DW answers some of the most common questions about COVID-19 vaccines. As new virus variants emerge, the efficacy of current vaccines could change.

A medical professional prepares a COVID-19 vaccine
Across the world, doctors like this one in Bangladesh are vaccinating people against COVID-19Image: Md Rafayat Haque Khan/Zuma/picture alliance

Despite having had both shots of the Pfizer/BioNTech vaccine, 14 elderly people at a nursing home in the northwestern German city of Osnabrück have tested positive for the B117 variant of COVID-19 first seen in the UK.

"There were some completely asymptomatic cases and otherwise only mild courses," Burkhard Riepenhoff, a press officer for the Osnabrück District Office, told DW.

Testing positive for COVID-19 without symptoms or having a mild version of the disease is normal after vaccination — that's the vaccine working.

Can vaccinated people still test positive for COVID-19?

Vaccines can create two kinds of immunity — effective immunity or sterilizing immunity.

Sterilizing immunity is complete protection from a virus — no virus particles can get into any cell in the body, it cannot replicate, and transmission can be stopped.

"That's like the holy grail of vaccines," said Sarah Caddy, a clinical research fellow in viral immunology at Cambridge University. "But that is really hard to achieve. To stop any virus particles getting in is virtually impossible."

Effective immunity stops the virus from causing severe disease but does not stop you from being infected or transmitting the virus.

Most vaccines can limit how much virus gets into the body, but there will still be some virus replication. However, being vaccinated should result in enough antibodies to reduce how much the virus is replicating, Caddy told DW. "That's why we're seeing a nice reduction in disease, but we are still potentially seeing some virus replication, which is then detected as asymptomatic infection."

Can vaccinated people transmit the virus?

Currently there is not enough data to know if mRNA and vector-based COVID-19 vaccines can prevent or reduce transmission, according to the Robert Koch Institute (RKI).

Until that data is available, vaccinated people and the people around them should continue to follow the recommended protective measures, like wearing a mask, keeping their distance and regular hand washing, RKI stated.

Most vaccines can't prevent transmission, said Caddy, including the majority of vaccines that have been in use for a long time. "They can't induce sterilizing immunity, but they reduce transmission enough that it's sufficient to stop the virus in the population."

Can vaccinated people get sick with COVID-19?

Yes, it is possible to become ill with COVID-19 even after vaccination. But vaccination helps to prevent severe, life-threatening disease. If you are vaccinated, you are more likely to be asymptomatic or get a mild version of the disease. The Pfizer-BioNTech vaccine is 95% effective in preventing illness from the original strain, while Moderna is 94% effective and Oxford-AstraZeneca is 76% effective.

S. Africa vaccination campaign

Can the vaccine protect against the B1351 and B117 variants?

As new variants of the coronavirus emerge, the efficacy of current vaccines could change. 

The Pfizer/BioNTech vaccine is likely to be effective against the highly infectious B117 variant of the virus, even though its efficacy is slightly affected, according to scientists at Cambridge University. But the E484K mutation — first seen in the B1351 variant — substantially increases the amount of antibodies required to prevent infection. The data has not been peer-reviewed yet and involved only a small number of patients.

South Africa has decided to suspend the rollout of the Oxford-AstraZeneca vaccine after a study of around 2,000 people found the vaccine offered little protection against mild and moderate cases of COVID-19. 

Shabir Madhi, a professor of vaccinology at the University of Witwatersrand in Johannesburg, told DW that recently released scientific data shows the Oxford-AstraZeneca vaccine is only 22% effective against the 501YV2 (also known as B1351) coronavirus variant that is dominant across South Africa.

Read more: BioNTech-Pfizer vaccine likely to protect against highly infectious UK variant

Will the COVID-19 vaccine cause you to test positive?

The Pfizer-BioNTech, Moderna and Oxford-AstraZeneca vaccines cannot cause you to test positive for COVID-19 on viral tests, which are used to see if you have a current infection. The vaccines do not contain the coronavirus itself. 

If your body develops an immune response, which is the goal of vaccination, it is possible that you could test positive on some antibody tests, which tell you if you have had a previous infection, the US health agency Centers for Disease Control and Prevention (CDC) states.

A person gets vaccinated for COVID-19 in Germany
It's normal to experience some redness, heat and pain at the injection siteImage: imago images/Jochen Eckel

Can a COVID-19 vaccination give you COVID-19?

The Pfizer-BioNTech, Moderna and AstraZeneca vaccines cannot give you COVID-19 because the vaccines do not contain the coronavirus itself. 

The vaccines teach your immune system how to recognize and fight the virus that causes COVID-19, according to the CDC. Sometimes this can cause symptoms like fever, headache, fatigue and muscle aches, which should only last a few days.

More common is experiencing some redness, heat and soreness at the site where you got the injection.

"That's common to virtually every immune response," said Caddy. "And actually, people I know who have had the vaccine have been really pleased that it hurt a little bit after, because it shows your immune response is working — a little bit of an achy arm is fine; that's completely normal."

It can take a few weeks for the body to build up immunity to the coronavirus after vaccination, according to the CDC. This means it is possible for someone to become infected with the virus just before or after receiving their vaccination and still develop COVID-19. But this is only because the vaccine hasn't had enough time to build up protection in the body.