Pfizer vaccine and isolation independently associated with reduced household transmission of SARS-CoV-2

With the onset of the coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), many parts of the world entered a phase of stringent restrictions through non-pharmaceutical interventions (NPIs). These have ranged from social distancing measures to full-scale lockdowns and, while global vaccination efforts are still underway, have been the order of the day to help mitigate the load on healthcare services straining to care for hundreds of seriously ill patients gasping for oxygen.

Israel was among the leading proponents of COVID-19 vaccination as the way out of the current worldwide restrictions on social and economic interactions and activity. A new study, recently released as a preprint on the medRxiv* server, reports on the effects of the large-scale vaccination campaigns carried out in this country, relating to successful control of SARS-CoV-2 spread within households.

Background

Despite the rapid development and unprecedented approval of several COVID-19 vaccines, beginning with the Pfizer BNT-162b2 messenger ribonucleic acid (mRNA) vaccine, the true efficacy of this measure against the virus remains largely unknown. The current work was meant to uncover the effect of vaccination on transmission within a household.

What was the study about?

Based on the risk of infection, earlier studies have already shown repeatedly that there was an 80% reduction of viral infection risk after two doses of the BNT-162b2 vaccine. Transmission risks were almost halved three weeks after the first dose. Even when the infection is acquired, symptoms are almost always mild.

Since these figures came from indirect measurements rather than from following up the households involved, and also because they did not consider the effect of isolation precautions on transmission, the current study incorporated these evaluation measures.

This is the first study that examines both vaccination and isolation, as well as age, relative to the risk of infection and household transmission, simultaneously integrating the effects of community infections, tertiary spread in the household, inability to identify the true index case, household size and low infectivity of asymptomatic cases.

The researchers chose to analyze household spread because of the high risk of spread within a household, up to 32%, which allows the effects of vaccination and other interventions to be observed. Moreover, this area of research will help understand how household interactions should go forward following vaccination.

The researchers found that two doses of the BNT-162b2 vaccine are very effective at reducing the risk of infection as well as transmission. Also, case isolation is still an excellent way to stop the spread of the virus from an infected individual within a household, perhaps even when the household contacts are vaccinated.

How was the study done?

Israel experienced the third wave of the pandemic at the beginning of 2021, due largely to the rapid and very extensive rise of the alpha variant of concern. However, simultaneously the government offered the vaccine to all above 18 years. As early as March 22, 2021, over 60% of the population had received the vaccine.

Following up SARS-CoV-2 transmission over the households of over 12,500 healthcare workers, they enrolled approximately 200 of them (or their household members) who had been confirmed to be infected with SARS-CoV-2. The mean age was 32 years, with somewhat more females among the index cases than males.

About 90% of the ~190 index cases were teenagers or older, and only <10% had taken the vaccine. There were 24 pediatric infections, all in unvaccinated children. Of the ~500 secondary cases in the household, one in four were vaccinated, and most of them were also isolated.

What were the findings?

About half the unvaccinated teenagers or adults were isolated during the study period. Vaccination was not employed in any of the children, but about half of them self-isolated during this period.

In total, ~40% of the ~270 household contacts acquired secondary infection, with almost two in three of them becoming symptomatic. Breaking it up, two-third were symptomatic. Of about 110 unvaccinated contacts, 75% were infected, but only ~25% of those who were isolated but unvaccinated. Among those who were both vaccinated and isolated following contact, ~11% were infected.

Among children, two-thirds were infected if not isolated, but only one in three if they were isolated. Over 55% of children were asymptomatic after infection, vs. a quarter of adult/teenager cases.

What if the index case was vaccinated?

When the index case was not vaccinated, ~40% of the contacts were infected, but only half that percentage among contacts if the index case was vaccinated. Household size did not seem to play a role in the secondary attack rate.

Thus, the relative risk of being infected following contact was 0.19 among vaccinated non-child contacts who did not isolate (relative to the risk of infection among unvaccinated who did not isolate. If not vaccinated, isolation reduced the risk to 0.11, while if both were done, it was 0.07.

Isolation may therefore reduce infection risk even among vaccinated contacts.

With the same reference group, child contacts had a relative risk of 0.50 without isolation, vs. 0.16 if isolation was practiced. Transmission risk from vaccinated cases was 0.22 times that of unvaccinated cases.

The researchers estimated that with four people in the household, the risk of transmission is ~60% between unvaccinated non-child individuals, vs. 4% if both index case and contact are vaccinated. If the index case is unvaccinated and the contact is a child, there is a 36% risk, vs. ~10% from a vaccinated index case, or if the child contact is isolated.

What are the implications?

The approach used here covers a wide range of potential infections and varying infectivity of symptomatic vs. asymptomatic cases. The secondary attack rate of 75% between unvaccinated cases and contacts is quite high, compared to earlier studies which pegged it at 14-32%. This may be because the study was carried out when the alpha strain was dominant, with its high transmissibility. Secondly, the active follow-up ensured that all secondary infections were identified, unlike another earlier study on the alpha variant’s transmission, which relied on passive surveillance.

Children were found to be infected at lower rates than older contacts, while vaccination reduced the risk of infection by >80% beginning seven days after the second dose. This confirmed the protective efficacy of this vaccine against the alpha variant.

Though it is a little lower than other estimates based on the general population, at 97% and 94% against symptomatic and asymptomatic infection at corresponding time points, this may be accounted for by the higher rates of contacts within a household, as well as the careful detection of all infections in this setting.

The findings show that isolation reduces the risk of transmission and of infection in all household contacts, possibly even after vaccination and that the secondary attack rate from a vaccinated index case is halved relative to an unvaccinated index case.

Vaccination with two doses substantially reduces the risk of transmission and the risk of infection in households. Isolation from the index case while sleeping and eating provides a high level of protection to unvaccinated household members, whether they are adults/teenagers or children,” conclude the researchers.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.