Vaccines against the coronavirus protect the vaccinated, but also the rest of the family. It is the main conclusion of a study supported by a massive crossover of data from British households. The work shows that the few who became infected after being vaccinated cut the likelihood of spreading COVID at home by half. The research, which has used statistics from more than 300,000 households and almost a million and a half people, is the biggest demonstration that the vaccine is the best firewall against the spread of covid.
Taking advantage of the fact that the United Kingdom has been very advanced with the vaccine (34 million already with the first dose, 53% of its almost 67 million inhabitants; Spain is halfway, 27%), researchers from the UK Public Health Agency (PHE) have crossed the information from three databases. On the one hand, those of the positives confirmed by a PCR since the beginning of the year and until March. On the other, that of those vaccinated with the drugs from Pfizer-BioNTech or AstraZeneca-Oxford. And finally, they grouped everyone who lived under the same roof using their public health affiliation number and the UPRN, a register that identifies every house in the UK.
The PHE scientists wanted to see how vaccination affected within homes, since homes are probably the place where there is the greatest risk of contagion because they are closed spaces where people live closely together for a long time and without a mask. The efficacy of vaccines is more than proven, but what they were interested in was to see if, in addition to protecting the vaccinated, it also protected those around them. For this, they used a sample with 365,447 positives considered an index case or the first case produced in each house between January and March. They lived with another million people. They saw that within 14 days of testing positive, there were another 102,662 infections in homes, called secondary cases. It is impossible to know if each of them was infected with the first one, but given the limitations to mobility and the advance of vaccination, it is the most likely. This yields a rate of contagion within the home of approximately one third.
What happened to the vaccinated?
Of those 365,447 households with an index case, 24,217 of them tested positive days after being vaccinated. The figure drops to only 4,107 infected, 1.12% of the total, if you count from 21 days, the estimated time to develop immunity. It should be borne in mind that in the United Kingdom only one puncture is being administered, which could lower protection somewhat. On the other hand, none of the cases required hospitalization. So everything indicates that vaccines protect those who get them.
The next thing the researchers did was see if they were also protecting those around them. To do this, they compared the homes with a first vaccinated but infected case with those where there was someone infected but the immunizations had not arrived. In unvaccinated households, there was 10.1% of cohabitants who became infected in the 14 days following the index case (96,898 out of a total of 960,765 contacts). But in homes where the index case had already been immunized, the second cases were reduced to 6.06% (567 new positives from 9,363 contacts). In other words, vaccines reduce the probability that covid spreads within the home by more than 40%. The percentage rose to 49% if the vaccine was that of AstraZeneca and Oxford.
In the 365,000 British households in the sample with a positive test between January and February, 1.12% were infected three weeks after being vaccinated
Dr. Mary Ramsay, head of immunization at PHE, said in a note about these results that “vaccines not only reduce the severity of the disease and prevent hundreds of deaths every day, now we see that they also have an additional impact reducing the risk of pass the covid to others ”.
This impact on the spread of the coronavirus is confirmed by another piece of information revealed by this research: the more time had elapsed between the moment when the index case (the first in the chain) was vaccinated and when it was infected, its probability of infecting its relatives was less. Only under the 10-day lapse were their chances of spreading COVID in their home equal to that of those who had not been vaccinated.
Africa González Fernández, president of the Spanish Immunology Society, highlights that the study has been carried out with the majority of people vaccinated with only one dose. “It is foreseeable that with two doses the decrease in the transmission of the virus will be even greater, as well as that there will be fewer people infected secondarily”, says the also professor of immunology at the University of Vigo.
It is a possibility already pointed out by the authors of the work, who recall that in the United Kingdom only 7% of those vaccinated have received the two mandatory punctures of the drugs that are being used. In Spain, on the other hand, the guidelines of the laboratories are respected and 41% of those vaccinated have received both injections except for a small proportion who, by using Janssen’s drug, have been protected with only one.
“From other studies, we know that the viral load of the vaccinated is lower than that of the vaccinated, which determines the possibility of contagion”
Àngela Domínguez, working group on vaccination of the Spanish Epidemiology Society
Until now, it was not known just how vaccines affected the transmission of covid. By design, clinical trials only measured efficacy in vaccinated individuals and not whether they were no longer contagious. There are a couple of investigations in healthcare settings, one in scotland and another in Israel, which showed a significant reduction in secondary infections after vaccination. But none in an environment like home and even less so with so many houses analyzed.
For Àngela Domínguez, coordinator of the working group on vaccination of the Spanish Epidemiology Society (SEE), “the home, due to the depth and frequency of coexistence relationships, is an environment where infections are very frequent.” It also highlights that, apart from this work, little is known about the role of vaccines in transmission. “From other studies, we know that the viral load of the vaccinated is lower than that of the unvaccinated, which determines the possibility of infection,” adds Domínguez.
Peter English, former head of the public health committee of the British Medical Association, highlights the relevance of these results. “They add to our arguments to hope that vaccines really do contribute to herd immunity. Evidence was accumulating that vaccination would prevent people from becoming infected (and if they are not infected, they cannot transmit the infection), ”he says. And he adds: “This study shows that even if vaccinated people become infected, they are much less likely to be contagious and transmit the infection to other people.”
* The study, not peer-reviewed, may consult here (PDF)
Eddie is an Australian news reporter with over 9 years in the industry and has published on Forbes and tech crunch.