Why fully vaccinated people can get Covid

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Nurses watch a computer screen in Bogota, Colombia on February 18, 2021.

JUAN BARRETO | AFP | Getty Images

LONDON – People fully vaccinated against Covid-19 are highly protected from serious infection, hospitalization and death from the virus. But coronavirus cases among the fully vaccinated – so-called “breakthrough” covid cases – are still seen in those who received two doses.

While vaccinated people rarely get Covid in the US or Europe, breakthrough cases occur for a number of reasons, experts note.

First off, none of the vaccines used in the US or Europe are 100% effective at preventing infections.

In addition, new Covid strains such as the highly contagious Delta variant – which is now widespread worldwide – have made the efficacy picture more difficult. There is also incomplete data on how long immunity to Covid lasts after vaccination.

The alarm was raised over groundbreaking Covid cases when preliminary data released in late July in Israel – which had one of the fastest vaccination programs in the world – showed that the Pfizer-BioNTech Covid-19 vaccine was only 40.5% effective at preventing symptomatic illness was.

The analysis, conducted when the Delta variant became the dominant tribe in the country, nonetheless found that two doses of the shot offered strong protection from serious illness and hospitalization, the country’s health ministry reported.

The data also appeared to show declining effectiveness of the Pfizer BioNTech shot, with the vaccine being only 16% effective against symptomatic infections in those who received two doses of the shot in January. However, in people who had received two doses by April, the rate of effectiveness (against symptomatic infection) was 79%.

However, a study conducted in England from April to May found that after two doses, the Pfizer BioNTech vaccine was 88% effective against symptomatic diseases caused by the Delta variant.

However, comparing the results is difficult given the differences in the nature of vaccination programs in the two countries (for example, Israel has given the Pfizer vaccine to the entire adult population, while in the UK several vaccines with the Pfizer BioNTech shot mostly at younger people) as well Differences in study dates, Covid test regimes and age groups.

Like the Israeli data, the English data concluded that the Pfizer-BioNTech Covid vaccine was 96% effective against hospitalizations from the Delta variant after two doses. Similarly, after two doses, the Oxford-AstraZeneca vaccine was found to be 92% effective in preventing hospitalization.

Initial efficacy data from clinical trials published by Pfizer and BioNTech last year for the vaccine showed that the vaccine was 95% effective against infections caused by strains of the virus circulating at the time.

Read more: Moderna could be superior to Pfizer compared to Delta variant – breakthrough chances increase over time

Professor Lawrence Young, a virologist at the University of Warwick Medical School in the UK, told CNBC that cases of Covid in fully vaccinated people are a reminder that “no vaccine is 100% effective”.

“There will always be a proportion of people who are still susceptible to infection and disease,” he said on Monday.

“There are also two other factors that affect the effectiveness of the vaccine: (1) Waning immunity – we still don’t know how long the protective immunity induced by the vaccine will last. This is very likely a factor in older and more vulnerable people who vaccinated at the beginning of the vaccine rollout program, “he noted.

The second factor, he added, relates to “breakthrough infections in vaccinated individuals due to the more contagious Delta variant,” which made the case more important for booster programs, he said. In the case of booster programs, the jury has not yet made a decision, in the USA and Great Britain a decision has yet to be made

Breakthrough cases by number

It’s difficult to know the full extent of the “breakthrough” Covid cases, but figures from NBC News have shown that at least 125,000 fully vaccinated Americans have tested positive for Covid and 1,400 of them have died. Still, the 125,682 “breakthrough” cases in 38 states found by NBC News represented less than 0.08% of the more than 164.2 million people (and will be) fully vaccinated since the beginning of the year, or about every 1,300.

That is, the number of cases and deaths among the vaccinated is very low compared to the number among the unvaccinated. Health authorities, especially in the US, are urging unvaccinated people to register for a Covid vaccination.

Andrew Freedman, an infectious disease reader at Cardiff Medical School, UK, told CNBC that “breakthrough cases” are expected.

“The vaccines are very good at protecting against serious infections, hospitalizations, and death, but they are less effective at providing complete protection against infection, and we know that many people who have been fully vaccinated are still having delta infections in most cases get mild symptoms. ” “He said on Monday to CNBC’s” Squawk Box Europe “.

“What we don’t know is whether an additional booster actually increases protection and reduces infections with delta variants,” he noted.

It must be emphasized that studies show that fully vaccinated people are much less likely to contract Covid – or even contract the virus at all.

New research from the UK published last Friday showed that people who were double-vaccinated were three times less likely to test positive for the coronavirus than those who were not vaccinated.

Analysis of the PCR test results in the REACT-1 study – a large coronavirus surveillance program in the UK led by Imperial College London – also suggested that fully vaccinated people may also be less likely to pass the virus on to others than those who were not vaccinated, because they have an average lower viral load and therefore probably less virus shedding.

Professor Paul Elliott, director of the Imperial School of Public Health’s REACT program, said the results highlight both the benefits and the limitations of Covid vaccines.

“These results confirm our previous data, which show that both doses of a vaccine offer good protection against infection. But we also see that there is still a risk of infection as no vaccine is 100% effective and we know that some are double vaccinated. “People can still get the virus,” he said.

Steven Riley, professor of infectious disease dynamics at Imperial, said “breakthrough infections” need further investigation in fully vaccinated people, especially as parts of the world are grappling with the spread of the Delta variant.

“The Delta variant is known to be highly contagious, and as a result, we can see from our data and others that breakthrough infections occur in fully vaccinated people. We need to better understand how contagious fully vaccinated people become infected as this will help better predict the situation in the months to come, and our results will help build a broader picture of it. “