A new study shows vaccinated people are about 40% less likely to catch Covid, but if or when they do catch it they pose the same risk to the people close to them regardless of their vaccination status.
The study also confirmed that vaccinated immunity was falling within three months of vaccination. Presumably, if a vaccinated person is 40% less likely to catch Covid in the first place, then being vaccinated will reduce the odds of bringing the SARS virus home on any given day. But given that protection wanes so quickly and Covid has such a high exponential rate of spread, a temporary 40% reduction of the risk of catching the virus is not game-changing.
The Imperial College study shows that draconian rules isolating the unvaccinated from the vaccinated are not medically justified. Put another way, an unvaccinated person infected with Covid is no more likely to spread the virus than a vaccinated person.
The Imperial College study followed 621 people, and was unusually detailed in measuring the load curves of viral titres as they rise and fall. They found that when infected, both the vaxxed and unvaxxed reached similar peak levels of virus, which supports the idea that they are both just as infectious.
The need for early treatment
By measuring viral loads daily they discovered that the initial rise of the virus was extraordinarily rapid for the first three days until it peaked. This early replication rate of the virus would go on to determine the trajectory of the whole infection. People who had the fastest rise and highest peaks also had the longest declines. It seems that whatever it is that slowed the infection in some people in the early days also helped to clear the virus faster. The authors don’t expand on this, but many other studies show early treatment, and especially prophylactic treatment is the most useful.
It is madness to send people home without an early treatment kit, and madness not to give that kit to all the household contacts to use before they get infected. That was the extremely successful tactic used in Uttar Pradesh which largely eliminated the virus.
Vaccinated People Easily Transmit COVID-19 Delta Variant in Households: UK Study
A year-long study from the Imperial College London published in The Lancet on Thursday found that the Delta variant is still highly transmissible within a vaccinated population.
Their study, which surveyed 621 participants, found that of 205 household contacts of people who had the Delta infection, about 38 percent of household contacts who were not vaccinated tested positive, compared with 25 percent who tested positive among vaccinated household contacts.
“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,” Dr. Anika Singanayagam, co-lead author of the study, said in a statement.
Immunity from full vaccination also dropped in as little as three months, their research also found.
But the critical line from the paper talks about the “secondary attack rate” (confusingly known as SAR) which means the rate of infections the primary case causes.
We identified similar SAR (25%) in household contacts exposed to fully vaccinated index cases as in those exposed to unvaccinated index cases (23%). This finding indicates that breakthrough infections in fully vaccinated people can efficiently transmit infection in the household setting.
What about natural protection — The big invisible factor?
One of the major limitations of this study is that it doesn’t even mention previous infections or natural immunity at all. It’s like a blindness. Some of the unvaccinated and the vaccinated in the UK have already had Covid and have natural protection which appears to be better and longer lasting. So the statistics are pooled values. Are prior infections more likely in the unvaccinated or the vaccinated groups? If the number of prior infections is higher in the unvaccinated group the study will underestimate the benefits of vaccination for people with no immunity. But if prior infections are more likely in the vaccinated that will make the vaccine appear to be more protective than it is. They could have done antibody testing on both groups and found this out. Even though vaccinated people have antibodies to the spike protein, people with natural protection have antibodies to the nucleocapsid as well.
If someone was infected with Covid they would go on to infect about a quarter of their household. This seems rather low compared to studies on the original Wuhan variant in the early part of last year. The reduced spread is probably explained by both vaccination and natural protection. But people may also be changing behaviour and reducing their contact with the newly diagnosed cases.
Those viral load curves
The details on the rapid growth of the virus in the early days reinforces the need to do early treatment or even better — prophylactic care. Most people don’t get symptoms til after the viral load peaks — they won’t even know the virus is multiplying unless a close contact has tested positive. And that if early treatment can reduce the rate of growth of the virus, it will also likely reduce the length of the illness and infection.
These are log scale graphs, so even a small decline in the initial rate of rise could make the infection much less severe.
Viral loads with the delta variant declined slightly faster as did viral loads in the vaccinated compared to the unvaccinated.
From the paper itself:
Vaccination was found to be effective in reducing household transmission of the alpha variant (B.1.1.7) by 40–50%…
Our findings help to explain how and why the delta variant is being transmitted so effectively in populations with high vaccine coverage.
Here’s a complicated way of saying that vaccinated people who tested positive were more likely to be the ones who got vaccinated longer than 2 or 3 months ago.
The time interval between vaccination and study recruitment was significantly higher in fully vaccinated PCR-positive contacts than fully vaccinated PCR-negative contacts, suggesting that susceptibility to infection increases with time as soon as 2–3 months after vaccination—consistent with waning protective immunity.
In this case, about 1 in 4 contacts became infected, which was a lot higher than the 10% risk that past UK studies had found (probably because testing was inadequate).
Household SAR [Secondary Attack Rate] for delta infection, regardless of vaccination status, was 26% (95% CI 20–32), which is higher than estimates of UK national surveillance data (10·8% [10·7–10·9]). However, we sampled contacts daily, regardless of symptomatology, to actively identify infection with high sensitivity. By contrast, symptom-based, singletimepoint surveillance testing probably underestimates SAR, and potentially also overestimates vaccine effectiveness against infection.
So there were probably more silent infections in the UK that past studies didn’t find, that made the vaccines look more useful at stopping the spread of infections than they really are.
Older people had higher peak loads (which is probably part of the reason they get sicker from Covid):
We found no evidence of variation in peak viral load by variant or vaccination status, but we report some evidence of modest but significant (pp=0·95) increases in peak viral load with age.
It’s surprising that they didn’t find a higher peak load in Delta cases (as past studies have) but then, people with past natural infections and natural protection are more common now. Perhaps that brings the viral load down?
Another big unknown — how many vaccinated people even turn up for testing?
We found no evidence of lower SARs from fully vaccinated delta index cases than from unvaccinated ones. However, given that index cases were identified through routine symptomatic surveillance, there might have been a selection bias towards identifying untypically symptomatic vaccine breakthrough index cases.
hat tip to Scott of the Pacific
Anika Singanayagam et al (2021) Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study Lancet
SAR [Secondary Attack Rate]
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October 30, 2021 at 09:41PM