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Why do fully vaccinated people test positive for Covid?



Phoenix Suns basketball star Chris Paul, British Health Secretary Sajid Javid and US Olympic gymnast Kara Eaker have one thing in common: they have all tested positive for coronavirus despite being fully vaccinated.

No vaccine is 100 percent effective, so what researchers call "breakthrough infections" is always expected. In most cases, the symptoms are mild.

As a new increase in Covid-19 cases has collided with a global vaccination campaign that produces more than 200 meters a week, more people are asking, "How protected am I?"

How many fully vaccinated people are test positive?

While anecdotal accounts of breakthrough infections may make such cases feel widespread, real numbers have remained small and were generally in line with expectations, experts said.

"There is no such thing as a perfect vaccine… With Covid it is no different," said Professor William Schaffner, an infectious specialist at Vanderbilt University.

The yellow fever jab, for example, is widely understood as the most effective live virus vaccine ever invented, with a single dose generating long-lasting immunity in 98 percent of those vaccinated. But even that means that on average 2 percent of people will still be infected.

Phase 3 studies for most of the leading Covid-1

9 jabs showed an effect against symptomatic infection of more than 90 percent. Actual efficacy studies in the United Kingdom, Israel and Canada suggest that vaccines show a somewhat lower efficacy outside the experimental environment, probably due to the spread of the more vaccine-resistant Delta variant. Estimates put protection against symptomatic infection, depending on the vaccine, at between 60-90 percent.

According to Public Health England, around 17 per cent of 105,598 cases of the Delta variant were reported across England during the four weeks to 19 July. among fully vaccinated people. PHE counts people as fully vaccinated 14 days after the second dose.

Anthony Masters, a member of the UK's Royal Statistical Society, said that fully vaccinated people were likely to account for a "greater proportion" of cases as vaccine coverage was expanded, especially in younger groups at higher risk of exposure due to greater social mix.

"If you get extremely high coverage at different ages, it is likely that cases may become [in] majority among fully vaccinated people," he said. Around 55 per cent of the British population had received both doses by 21 July.

In Israel, where almost 60 percent of the population is fully vaccinated and coverage is more evenly distributed by age group, 52 percent of around 6,000 people who tested positive in the week to July 21 were fully vaccinated.

Are some fully vaccinated people at greater risk of becoming ill than others?

Very few fully vaccinated people who test positive for Covid-19 become seriously ill. According to PHE's actual studies, the BioNTech / Pfizer vaccine is still 96 percent effective against hospitalization, while the Oxford / AstraZeneca shot is 92 percent effective.

But Natalie Dean, a biostatistics professor at Emory University in Atlanta, emphasized that these numbers were average and that the effect depended on people's existing risk profiles. "Everything is relative when it comes to vaccines and risk," she said.

For example, a Financial Times analysis of global deaths from infections suggests that an 80-year-old with a double jab now faces the same mortality risk. as an unvaccinated 50-year-old.

  Diagram showing that vaccines have made Covid-19 far less lethal. A fully vaccinated 80-year-old now has the same mortality risk as an unvaccinated 50-year-old

In England, where vaccine roll-out has been shifted from oldest to youngest and nine out of ten over 50-year-olds have been fully vaccinated, 30 per cent of the 1788 the people who were hospitalized due to the Delta variant in the four weeks to 19 July, fully vaccinated. About half of the 460 deaths in the country associated with the Delta strain since February were people who were also fully vaccinated.

"This is simply a reflection of the fact that vaccine intake is very high among older people," said Masters. "It is perversely a marker for [a] successful rollout. If everyone [was] were fully vaccinated, everyone who went to hospital or died would by definition be fully vaccinated."

About two-thirds of people who die on roads in Britain use seat belt, but this is a consequence of the use rate of almost 99 percent, Masters said, adding that the same logic applied to serious illness and death in highly vaccinated populations.

Vanderbilt's Schaffner added that people who experienced unpleasant but mild symptoms , probably would have had a serious illness, or even death, had they not been vaccinated. "When my patients tell me they still had a mild illness despite being vaccinated, I always say I'm glad you're still here to complain. "

  Diagram showing why a high proportion of double-jacketed hospital admissions do not mean vaccines fail

Can you test how protected you are?

Not yet . The easiest way to understand how much immunity the vaccine has generated in a person is to measure the presence of neutralizing antibodies in the blood. But T cells and B cells, which supplement the body's immune system, also play a role, and researchers are still unclear about which scales provide the best insight into the vaccine's effectiveness.

Commercially available antibody tests only show if an individual has Covid-linked. antibodies or not, but immunity is best understood as a "scale or continuum", explained Danny Altmann, an immunology professor at Imperial College London. “It is not binary. You are not safe or insecure, protected or unprotected. Humans all have varying degrees of protection against the vaccine.

This area of ​​immunity can be plotted with a test called a neutralization assay, which analyzes how many times antibodies taken from the blood can be diluted in a laboratory and still neutralize viruses.

At the extremes, immunocompromised humans can only generate enough antibodies to withstand a 100-fold dilution, Altmann said. In comparison, healthy young people may have enough to dilute 10,000 times and are "probably impenetrable" to infection.

If scientists were able to establish the midpoint between the two extremes, Altmann said that "vaccine manufacturers would be able to update vaccines faster for new variants and decision makers could better determine which people are most in need of booster doses."

What does imperfect vaccines for herd immunity?

Kit Yates, a mathematical biologist at Bath University, warned that the imperfect protection offered by the vaccine against infection meant that herd immunity could be "impossible" without vaccine over 90 percent.

" Leaking vaccines are likely to put herd immunity out of reach, especially when faced with [the] Delta [variant]"he said.

Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, said that with the UK expecting over 100 000 cases per day by the end of August, the implications of imperfect vaccines "will soon become clear".

PHE estimates that in average Covid-19 vaccines used in the UK are between 91 and 97 per cent effective in preventing hospitalization.

Kucharski warned that small differences could have a major impact on how far this wave of infections stretches the British healthcare system. "If you reverse the number, you are left with how ineffective the vaccines are, and 9 percent ineffective rather than 3 percent ineffective means three times more hospital admissions."


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