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New research finds that vaccination is not enough to protect against Delta variant transmission and highlights the importance of mask-wearing. Maite Pons/Stocksy
  • Researchers conducted a study to investigate the transmissibility of the SARS-CoV-2 Delta variant among vaccinated individuals in the same household.
  • They found that while vaccinated individuals may be less likely to get the infection, they are just as likely to pass on the virus if they contract it.
  • The researchers also found evidence that vaccine protection may wane after 2–3 months of being vaccinated.
  • They conclude that vaccines alone are insufficient to contain the Delta variant, people should maintain non-pharmacological precautions such as mask-wearing, and those eligible for booster shots should get them promptly.

COVID-19 vaccinations significantly reduce a person’s chance of developing adverse outcomes and dying from SARS-CoV-2 infection.

In England, research has also found that getting fully vaccinated reduces transmission of the Alpha variant by 40–50% in households.

The same study found that if they acquire the infection, individuals also have a lower viral load in their upper respiratory tract than those who are unvaccinated.

However, the Delta variant (B.1.617.2) has replaced the Alpha variant as the dominant variant worldwide. Current vaccines remain highly effective at preventing hospitalization and death from infection with the Delta variant.

But vaccines are less effective against the Delta variant than against the Alpha variant. The Delta variant also continues to cause a large number of cases in countries with both low and high vaccine coverage.

There has so far been little research into the risk of community transmission of the Delta variant from vaccinated people with mild infections. Understanding this could help policymakers improve guidelines to curb the COVID-19 pandemic.

Recently, researchers led by Imperial College London and the University of Oxford collaborated on a study investigating the transmissibility of the Delta variant among vaccinated individuals within households.

“Understanding the extent to which vaccinated people can pass on the Delta variant to others is a public health priority,” says Dr. Anika Singanayagam, co-lead author of the study. “By carrying out repeated and frequent sampling from contacts of COVID-19 cases, we found that vaccinated people can contract and pass on infection within households, including to vaccinated household members.”

She adds: “Our findings provide important insights into the effect of vaccination in the face of new variants, and specifically, why the Delta variant is continuing to cause high COVID-19 case numbers around the world, even in countries with high vaccination rates. Continued public health and social measures to curb transmission — such as mask-wearing, social distancing, and testing — thus remain important, even in vaccinated individuals.”

The study appears in The Lancet Infectious Diseases.

Between September 2020 and September 2021, the researchers identified 621 participants via the National Health Service (NHS) Test and Trace, the contact tracing system in the United Kingdom.

These included 19 cases of initial infection and 602 people who either lived with or had contact with symptomatic individuals with confirmed COVID-19 status. Children as young as 5 years old were able to participate with parental consent. However, the cohort’s median age was 36 years old.

Each participant underwent daily PCR tests for 14–20 days to track the progression of their infection. This allowed the researchers to detect changes over time in their viral load, or the amount of virus in a person’s nose and throat, and enable comparisons between fully vaccinated, partially vaccinated, and unvaccinated individuals.

The researchers detected SARS-CoV-2 RNA in 163 (26%) participants, and whole-genome sequencing of PCR-positive cases confirmed that 71 had contracted the Delta variant, 42 the Alpha variant, and 50 had pre-Alpha variant infections.

All of the participants had mild symptoms or were asymptomatic, and the proportion of asymptomatic cases was similar among all participants, whether vaccinated, partially vaccinated, or unvaccinated.

Among 205 individuals who had household contact with those confirmed to have the Delta variant, 53 tested positive for COVID-19.

A quarter of household contacts who had received two doses of the vaccine acquired infection with the Delta variant, while the same was true for 38% of unvaccinated household contacts.

The researchers estimated that vaccines were 34% effective — regardless of symptoms — at preventing transmission of the Delta variant in household settings.

The average time interval between vaccination and study recruitment for fully vaccinated individuals with PCR-positive results was 101 days. For fully vaccinated individuals with PCR-negative results, it was 64 days. This, say the researchers, suggests that immune protection from vaccines may wane in as little as 2–3 months following vaccination.

“Vaccines are critical to controlling the pandemic, as we know they are very effective at preventing serious illness and death from COVID-19,” says Professor Ajit Lalvani, co-lead author of the study. “However, our findings show that vaccination alone is not enough to prevent people from being infected with the Delta variant and spreading it in household settings.”

He continues: “The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe COVID-19, especially as more people will be spending time inside in close proximity during the winter months. We found that susceptibility to infection increased already within a few months after the second vaccine dose, so those eligible for COVID-19 booster shots should get them promptly.”

The researchers also found that among those who acquired any variant of COVID-19, viral load declined most rapidly among those who were fully vaccinated and least quickly among those who were unvaccinated. They noted, however, that vaccinated people recorded similar peak viral loads to unvaccinated people.

The speed at which vaccinated people can reduce their viral load explains how being vaccinated reduces hospitalization. However, similar peak viral loads demonstrate how the variant can still spread despite vaccination, as a high viral load indicates a higher level of transmissibility.

“It’s been known for a few months now that fully vaccinated people can become infected within the household setting, something attributable to both waning immunity — against infection much more so than disease — and the circulation of the more transmissible Delta variant,” Dr. John P. Moore, a professor of microbiology and immunology at Weill Cornell Medical College, not involved in the study, told Medical News Today. He went on:

“The results [of this study] don’t seem particularly surprising, but they add to what’s known. Thus, within households, virus spread can occur even if everyone is vaccinated, but less so (by approximately 50%) than when the household members are unvaccinated. One reason for the difference is that viral loads decline more rapidly from the peak in the vaccinated people, reducing the chances of onward transmission.”

Dr. Moore told us: “One caveat when extrapolating to the [United States] is that only 14 of the 38 Delta breakthrough infections in vaccinated people involved the Pfizer vaccine. The rest had received [the Oxford–AstraZeneca vaccine] (23) or the [Sinovac vaccine] (1), which are both less effective than the mRNA vaccines that predominate in this country. Hence, here, I would expect the vaccination benefit to be greater than the numbers reported in this U.K. study.”

The researchers conclude that vaccines alone are insufficient to prevent the spread of the Delta variant in households. They say that both vaccination and non-pharmacological interventions like mask-wearing will remain crucial for containing the pandemic.

One limitation to the study is how the researchers defined who may have contracted SARS-CoV-2 first in a household. It is possible that another member of the household may have had the infection before the person whom the tracking system identified.

Another limitation is that as older people received vaccinations before younger people in the U.K., any age-related findings may be skewed.

“This study confirms that whether vaccinated or not, once a person is infected with [SARS-CoV-2], they can [pass it on] to others. Vaccinated people who develop COVID are likely less [prone to pass it on], though,” Dr. Jorge Luis Salinas, an assistant professor of medicine at Stanford University, not involved in the study, told Medical News Today.

When asked what the study means for people deciding how best to protect themselves and their families, Dr. Salinas said: “Vaccines work. They prevent infections. When an infection happens in a vaccinated person, they likely have and spread less virus than unvaccinated people.”

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