Can vaccinated people still catch and spread the disease?

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Yes: vaccinated people can catch an infection and pass it on, including to other vaccinated people. This does not mean vaccines offer no protection. Studies have found fewer cases with symptoms and less spread in particular settings.

Why people believe it

The starting observation is real. A UK study documented spread between fully vaccinated household members. It studied Delta, a version of the COVID-19 virus. It is fair to ask what protection means if that can happen.

KFF surveyed US adults in September 2021. Of those surveyed, 26% chose the view that COVID-19 infections after vaccination meant the vaccines were not working. The other option was that most vaccinated people who became infected with COVID-19 did not require hospitalization, meaning the vaccines were working. Of those surveyed, 68% chose this option. Those were competing readings of the news, measured in 2021. They are not a poll of opinion today.

“You can still catch it”

Yes, but compare how often. In the 2020 BNT162b2 COVID-19 vaccine trial, researchers counted cases with symptoms. They counted eight cases in vaccine recipients versus 162 in placebo recipients. These cases had symptoms beginning at least seven days after dose two. These counts were among 36,523 participants without evidence of existing or previous SARS-CoV-2 infection. This measured illness with symptoms and a positive test. The initial report did not answer whether vaccination prevented infection without symptoms.

The disease matters too. MMR is the measles, mumps and rubella vaccine. CDC says two doses are 97% effective against measles and 86% effective against mumps. One percentage cannot stand for every disease.

“You can still pass it on”

Yes. The UK Delta study’s abstract, its short summary, reported similar household infection rates, with wide uncertainty ranges. The rates followed exposure to an already infected person. They were 25% when that person was fully vaccinated, versus 23% when unvaccinated. Those figures describe spread from people already infected. They do not measure vaccination’s effect on the chance of becoming infected in the first place.

A Danish study looked at 24,693 households during Delta in 2021. It compared fully vaccinated and unvaccinated primary cases, the first people to test positive in each household. It estimated that fully vaccinated primary cases were 31% less infectious to unvaccinated household contacts than unvaccinated primary cases. Its 95% confidence interval, a measure of uncertainty, was 26% to 36%. These findings do not show one universal reduction in how easily people spread infection.

“The protection wears off”

It can. A review looked at 18 studies, all before the Omicron version of the virus circulated widely. It compared months one and six after full COVID-19 vaccination. In its all-ages analysis, it estimated that protection against infection fell on average by 21 percentage points, meaning the difference between percentages. In the same all-ages analysis, the estimated average fall against severe disease was smaller: 10 percentage points. The review rated seven studies at serious risk of a distorted result. Declines can reflect weakening immune protection, a new version of the virus or distorted study results. The review did not pin everything on one cause.

“Then how can it protect other people?”

WHO defines herd immunity as indirect protection from an infectious disease when a population is immune through vaccination or previous infection. CDC says vaccinating a high share of people against measles also protects people unable to be vaccinated. That is not a guarantee for each person. Intense or prolonged exposure can still lead to measles after vaccination. The idea does not apply to every disease. Tetanus has no herd immunity because it does not spread between people.

Official positions, with dates

WHO’s global guidance is dated 15 September 2026. It recommends routine COVID-19 vaccination for people at highest risk of severe disease. CDC’s US page was updated 8 October 2026. It recommends a 2026–2027 COVID-19 vaccine for all adults aged 18 and over. These are the bodies’ recommendations, with different scopes.

What people also ask

Do unvaccinated kids affect vaccinated kids?

They can: prolonged close contact with someone with measles can lead to infection despite vaccination. High community measles vaccination coverage also protects people unable to be vaccinated.

Does any vaccine provide 100% immunity?

WHO says vaccines do not provide complete protection for many reasons. These include differences in how people’s immune systems, the body’s defenses, respond.

Can I get measles if I was vaccinated 30 years ago?

Infections after vaccination can occur. But CDC considers two properly scheduled childhood doses lifelong protection without an extra dose. For people with a history of a weakened immune system, CDC says a specialist should assess any need for vaccination again. The specialist should also assess its timing.

Can an unvaccinated child infect a vaccinated child?

Yes, if the first child has the infection. CDC says vaccinated people can still catch measles, mumps or rubella when exposed.

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