COVID-19 vaccination reduces household transmission up to 97%

Households have proved to be one of highest risk settings for COVID-19 transmission, particularly since the emergence of Delta.
 
This trend has raised concerns for the unvaccinated and immunocompromised, leading New South Wales to restrict home visits to the fully vaccinated, while in Victoria, home visits will be one of the last freedoms to be restored.
 
But new research out of Sweden has offered some reassurance; it suggests that people without immunity have an increasingly low risk of contracting COVID-19 in the household as the number of immune people living there increase.
 
Published in JAMA Internal Medicine, the study involved 814,806 households of 2–5 people and looked at the likelihood of unvaccinated household members becoming infected in the context of how many people in the household have immunity to COVID-19, either through vaccination or previous infection.
 
The results showed that compared to a fully unvaccinated household, an unvaccinated individual living with one person with immunity has a 45–61% lower chance of contracting the virus.
 
When living with two immune people that risk dropped further to 75–86%, with three it reduced to 91–94%, and when all four other household members were immune, the risk of infection was 97% lower.
 
Vaccinated people included in the study had been vaccinated with either AstraZeneca, Pfizer or Moderna, all of which are being used in Australia.
 
However, the majority of cases studied had been caused by the Alpha variant, which has a reproductive rate (R0) of 3.5–5.2 compared to Delta, which has an R0 of 3.2–8.
 
Nonetheless, Professor Adrian Esterman, Foundation Chair of Biostatistics at the University of South Australia and former World Health Organization (WHO) epidemiologist, told newsGP the findings still have implications for Australia.
 
‘It’s almost implying that within a family, you can actually start approaching herd immunity the more family members who are immune, and so the less chance there is of non-immune ones getting infected,’ he said.
 
The findings also showed that the benefit of immunity acquired from a single dose, in lowering transmission within families, was similar to the benefit from full vaccination or a previous infection.
 
Professor Esterman believes this is contributing to the trend emerging in New South Wales, which has seen infection numbers decline dramatically from its 10 September peak, where daily cases topped 1600 and less than 45% of the population was fully vaccinated.
 
One month on, the state’s five-day moving average is now down to 492 daily cases, and the effective reproduction number (Reff) is down to 0.80.
 
As of 12 October – a day in which the state recorded 360 locally acquired cases – 74% of people aged 16 and older were fully vaccinated and 90.4% have received at least one dose.
 
Meanwhile, Victoria’s vaccination rates are not as high, with 85.8% having had a first dose and 59.3% fully vaccinated, but a similar downward trend is also emerging.
 
More than 14560 cases were recorded on the day of publication, but the five-day moving average has reduced to 1743 – the first drop in almost seven weeks – and the Reff is now down to 1.10.
 
And without vaccination, Professor Esterman’s projections based on the current epidemic curve had Victoria’s daily count at 9000, indicating it may be playing an even bigger role down south than in New South Wales.
 
‘People are saying “Oh, this is terrible; we’re getting up to 2000 cases a day in Victoria”, but without vaccination, it was probably going to be more like 9000 cases a day,’ he said.
 
‘So it’s made an absolutely huge difference – and you’d probably expect it to as well because clearly, as more and more people are vaccinated, there’s far less chance of transmission and the effective reproduction number comes down, which is exactly what we’re seeing.’
 
But will New South Wales’ easing of restrictions this week have implications, and soon be reflected in the numbers? Professor Esterman suspects it will.
 
With Delta’s median generation interval – the time it takes from a person being infected to infecting someone else – standing at around five days, he anticipates any impact of opening up will show by the weekend. 
 
But that’s not to say that it will be reflected in hospitalisation rates.
 
In the UK experience, where up to 40,000 cases are being confirmed per day, hospitalisations are hovering around 740.
 
‘So it’s a 2% hospitalisation rate,’ Professor Esterman said.
 
‘We’re starting to get quite a big disconnect between case and hospitalisations in Victoria and we’re also seeing it, but to a lesser extent, in New South Wales and I think [that disconnect] will start increasing even more over the next few weeks.’
 
This increase in case numbers in contrast to hospitalisation rates is in large part due to older people being among the highest vaccinated age groups, resulting in a reduction in the mean age of infection, with the majority of cases now among those aged 20–29.
 
‘And, of course, younger people don’t get so sick,’ Professor Esterman said.
 
‘So I think that’s what we’re seeing here, and what is happening now in Victoria, and almost certainly will happen the next few weeks in New South Wales.’
 
While the Australian Technical Advisory Group on Immunisation has given the go ahead on a third dose for people who are immunocompromised, some concerns remain for children aged under 12 who will see out eased restrictions despite a vaccine not yet being approved for the younger cohort.
 
But Professor Esterman says the Swedish research provides reassurance, both for concerned parents and households with individuals who cannot be vaccinated.
 
‘Younger kids rarely get sick [from COVID] and, I don’t think we’re 100% sure, but probably can’t transmit it as much because they’re not coughing or sneezing,’ he said.
 
‘So it’s reassuring that this is not a major issue, as long as you have one or two people in the family who are vaccinated – that’s a very important message.
 
‘And also, we must never forget that this is just the first generation of vaccines. Over the next 12 months, we’ll see second and third generation, which will be much more targeted towards variants, and on top of that, we’ve now got a really good treatment for COVID-19. So there’s lots to be optimistic about.’
 
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