Do vaccinations stop the spread of Covid-19?

With infection rates soaring, the rollout of the Oxford-AstraZeneca vaccine couldn’t have come soon enough. But a crucial question remains unanswered: are vaccinated individuals still able to transmit the virus?

While the AstraZeneca trial results were a little patchier than those of Pfizer and Moderna, they have still yielded immensely positive results: there have been no serious cases of illness from Covid-19 among groups who received the vaccine. However, crucially, the studies for all three vaccines have focussed on illness and symptoms. We know that these vaccines stop you from falling ill but we still don’t know if they prevent infection. In other words, we are lacking data on whether or not vaccinated individuals can be asymptomatic spreaders.

The science on asymptomatic virus carriers has been a source of contention throughout the pandemic. The level of asymptomatic spread which occurs is still unclear. It’s now thought to be much lower than initially predicted (an oversight caused by earlier studies confusing “asymptomatic” and “pre-symptomatic” patients). But even recent studies provide very vague figures. According to Nature, asymptomatic spread could be anywhere from 3 to 25 times lower than symptomatic spread. Most scientists do agree however, that, unlike with SARS, asymptomatic spread is possible with Covid-19.

The question of whether or not vaccinated individuals can be asymptomatic carriers has major implications for herd immunity.  A vaccine which blocks infection – and prevents transmission – will achieve herd immunity much more readily and likely avert many more deaths than one which only prevents illness. And, while the latter option still provides immense relief, it’s crucial to understand whether or not vaccinated individuals pose a risk to (non-vaccinated) others.

AstraZeneca has indicated that its vaccine may indeed curb transmission but the pharma giant has provided little by way of detail. In November, AstraZeneca stated that there had been an “early indication that the vaccine could reduce virus transmission” thanks to “an observed reduction in asymptomatic infections.” Katie Ewer, an immunologist working on the Oxford vaccine told Nature that they had also identified this reduced infection rate within the vaccinated group (compared to the placebo group) by making UK trial groups take regular Covid-19 tests.

So far, Pfizer and Moderna’s study results have focussed solely on the vaccine’s ability to prevent illness. When it comes to curbing infection, the results provide little by way of answers. But this is set to change.

Jerica Pitts, a spokesperson at Pfizer, told WIRED that “in the coming months”, Pfizer will analyse blood samples from its trial participants. They will look for antibodies that recognize a part of the virus that is not in the vaccine. “If fewer participants in the vaccine group than in the placebo group develop such antibodies,” she explains, “we will have evidence that the vaccine can prevent infection as well as disease.”

Moderna plan to go through the same process. “It will take several weeks before we can expect to see those results,” Colleen Hussey, a spokeswoman for Moderna, told the New York Times.

Ongoing clinical trials for the Novavax vaccine (purchased by Australia) are also encouraging. These trials have taken transmission into account and, so far, they indicate that the vaccine does protect against infection as well as illness, although it would of course be unwise to assume these results are automatically applicable to other vaccines. In fact, potential discrepancies in levels of infection protection could have major implications for the future desirability of competing vaccines.

Mass vaccination will facilitate epidemiological studies on herd immunity– and hopefully shed more light on the vaccine’s ability to prevent transmission. But more ambiguities no doubt lie ahead: for instance, we will need to distinguish between a reduction in Covid-19 cases caused by the vaccine rollout and those potentially caused by the impending national lockdown.

Piecing together what we know so far, it seems that vaccinated individuals are almost certainly at a reduced risk of transmission. But establishing the degree to which this holds true will be essential to developing an effective vaccination strategy in the coming months.