Do Covid19 vaccines stop the disease spreading?

Vaccines work by preventing infections altogether or by reducing the severity of disease. But do the new COVID19 vaccines also prevent us from passing on the disease to others?

The three main international COVID19 vaccines have all published their data: Moderna and Pfizer were published in the New England Journal, and Oxford & Astra-Zeneca was published in the Lancet. All three now have emergency use authorisations in the UK and some other countries have also approved at least one of them. Covid-19 vaccines seem to be very good at reducing the severity of the disease. I have in a previous article outlined that in the clinical trials hospitalisation and death did not occur in anyone more than a few days after they received their first vaccine dose. Of course we can’t be certain that this 100% success rate will also be true when the vaccine is used on people with significant co-morbities and in particular immune suppression (such as is seen in blood cancers). But the hope is that the vaccines will indeed have a strong effect in the real world.

What is the evidence for the use of COVID19 vaccines?

Prevention rates of symptomatic COVID19 in some cases were also higher than 90%. This means that only 10% of the people who would normally catch COVID19 will do so if they are vaccinated. But that 10% who do catch the disease generally get it much less severely. The percentage may possibly higher depending on how you define symptomatic COVID-19 and which vaccine you are talking about. Clearly changing the disease from having the potential to cause a severe pneumonia which needs hospitalisation to more like a common cold or flu is a huge success. When you see figures for vaccine efficacy it does NOT mean it is for example 90% effective in everyone, it rather means it will work 100% in say 90% of people. The remaining 10% of people would not be prevented from catching COVID19 if exposed, but current evidence suggests that those few people who it had not worked for would not normally get severe disease.Close to 100% of people in the studies made antibodies even after the first dose. The extent of all the measures of efficacy were remarkable. This all seems to imply if you have antibodies in your blood when you are exposed to COVID19 you almost certainly won’t get hospitalised and most likely won’t even get sick. This has given encouragement to the concept of monoclonal antibodies for those who may not respond to the vaccine.

Volunteering for a monoclonal antibody COVID19 clinical trial

Can people who have been vaccinated still get asymptomatic disease? And if so will they potentially spread the disease to others who are either unvaccinated or their vaccine did not work? This question is especially crucial for those of us with blood cancer as may be at a higher risk of not making antibodies to the vaccine, and a higher risk of severe disease if we do get exposed.

High risk for Blood Cancer patients with COVID19

Many doctors are willing to check antibody levels a few weeks after vaccination for people with blood cancer to see if the vaccine has worked. I would personally urge all our readers with blood cancer to try to have this test done to help them understand whether their body has made any response at all to the vaccine. I recently had a test vaccine for a different disease and made no antibodies whatsoever. But many people with blood cancer will make antibodies, just please do not assume you are one until you are tested. As a finer point you may have some protection via T cells even without antibodies but that is harder to test for. But back to the main question which for those of us who may not respond to the vaccine could be rephrased: “Do our vaccinated friends and family still pose a risk to us?”

Do the vaccines prevent asymptomatic cases (and hence disease spread)

Unfortunately the Pfizer study did not look at assymptomatic disease at all. They did not take swab tests unless they had symptoms. This does not mean that the Pfizer vaccine does not reduce assymptomatic disease and hence spread, it just means we do not know what proportion of vaccinated people might carry the virus asymptomatically.The Moderna study was not designed to examine the incidence of asymptomatic cases although they did do a swab in every patient prior to the second injection. 39 (0.3%) in the placebo group and 15 (0.1%) in the mRNA-1273 group were assymptomatic for COVID19 but had swabs that were positive by PCR at the second dose visit (surveillance swab). It is perhaps unfortunate that this study did not have additional surveillance swabs, but this solitary data point does suggest that there might be approximately a 30% reduction in the risk of asymptomatic disease in the placebo group. If this real reduction in asymptomatic spread really is much lower than the reduction in symptomatic spread this might suggest that the primary effect of the vaccine is to reduce the severity of infections. Note that this apparent reduction in asymptomatic spread occurred after only one dose. And so we do not have clear evidence currently that the Moderna vaccine completely prevents asymptomatic transmission (i.e. some patients who had the vaccine still catch COVID19 and pass it on but not actually get sick. This would mean that the expected effect of reducing transmission of the disease will take longer, and indeed it might even mean that it is not feasible to completely eradicate COVID19 from the population.The Oxford & Astra-Zeneca study was unique in that it did aim to assess the incidence of asymptomatic spread. Like the other vaccines it had a 100% success rate in preventing the most severe cases of COVID-19 infection beginning a few days after the first dose. It also reduced the rates of symptomatic COVID-19. But what of the swabs conducted on people without symptoms?Across the entire population a remarkably similar finding to the limited Moderna data was seen. 0.9% of vaccinated participants experienced a positive swab in the absence of symptoms, compared to 1.2% of those who were in the control group. This corresponded to a reduction of risk of 27.3%, meaning that the risk of getting asymptotic disease was reduced by around a third. This rose to a reduction in risk of just under 60% in the group of patients who received the low dose first followed by standard dose. Many of these patients also had a longer interval between doses so the greater efficacy seen may be due to either reason or a combination.The reduction in asymptomatic disease seen in both the Astra-Zeneca and Moderna studies is enough to make a big difference in transmission rates in a population. But it still means that there remains a significant risk that any individual if exposed to COVID19 may still be able to pass on the disease, if they do not develop symptoms. This is likely to also be the case with those who have developed immunity from previous infection with COVID-19 too. Unfortunately if asymptomatic infection is not reliably prevented in every vaccinated person this would mean that blood cancer patients remain at risk of infection if they had not responded to the vaccine themselves. The lack of clear evidence for a full shielding affect of the vaccines on transmission is why, unlike the flu vaccine, at the moment the UK government is not prioritising vaccinating people who live with those with an impaired immune system.The most important risk indicator for those who are concerned about their own level of immune response and vulnerability to severe COVID19 will be the background level of COVID19 infection in the population (as measured by in the UK by the weekly ONS survey.The hope is, however, that by reducing the severity of disease, even if some people will get asymptomatic infection, spread will be reduced since the lower viral loads seen mean that you are probably less likely to pass on disease than if you are coughing all the time from symptomatic COVID-19.Right now at least here in the UK we are in a very dangerous phase of the pandemic. We know that around 1 in 50 people across the UK (much more than this in some areas) are currently positive for COVID-19. In most cases this is with the new more easy to transmit variant.The hope is that by vaccinating as fast as possible, even if the vaccines do not eliminate asymptomatic spread but merely reduce it, the rates will quickly come down. Those of us with blood cancer should watch the UK ONS figures or the equivalent in your own country very carefully as once those background rates have gone down the risk of us getting infected will also go down even if we personally do not respond to the vaccines. This new variant as well as others that are more easily transmitted are currently also spreading in other countries including the USA.The USA dont have the same random screening to see what the real population infection is but in some parts rhe hospitals are heaving