As vaccine distribution is picking up with nearly 70 million doses administered worldwide, data regarding large-scale vaccination is becoming available. Last week, Israel released preliminary data on the effects of vaccination for infection. Early indications suggest that after vaccination, as many as 70% of people can still be infected. More data is needed to confirm the effects on transmissibility among the vaccinated. 

Israel, which leads the world in vaccinations per capita at around 43 doses per 100 people, developed this study to understand how the Pfizer-BioNTech vaccine affects viral infection. Previous studies show the vaccine is effective in preventing Covid-19 related mild and severe symptoms. While the Covid-19 vaccines are intended to protect a patient from symptoms, having your shots doesn’t necessarily mean you can’t still be infected by SARS-CoV-2 or spread it to others.

The study contrasted a group of 200,000 vaccine recipients aged 60 and above with a group of 200,000 who hadn’t been vaccinated in the same age group. The researchers’ initial findings found that the two-dose Pfizer-BioNTech vaccine reduced infection chances by 33% two weeks after the first injection. 

The release of this data is a positive development. For months, the impact of vaccines on the pandemic has been mostly speculative. Nobody knew how mass vaccination might affect infectability or transmissibility. This data is the first of many, and we await data from similar studies.

While data remains limited on the transmissibility of the vaccinated, it may be the case that it is reduced among most. If neutralizing antibodies from the vaccines do their jobs, the viral load in a person infected with SARS-CoV-2 is much lower than in those who haven’t been vaccinated. A lower viral load may mean that they are less likely to infect others with the same efficiency that an unvaccinated person may. The vaccinated patient has less virus in them to spread around. Though we don’t know whether this is the case, more data is needed to know if those vaccinated and infected can transmit the virus to others.

A 33% reduction in risk means 67% remain at risk of infection. Vaccine protection from disease, but not infection, is observed for other respiratory viruses. The flu vaccine, for example, does not typically protect people from infection and further transmission, but only symptoms. Flu vaccines typically reduce risk by 40-60%, meaning receiving a flu shot won’t guarantee you protection from influenza infection.

The question of transmissibility is more complicated. Some studies have been conducted on guinea pigs for influenza, showing a moderate reduction in influenza transmissibility from vaccinated hosts. Infection in those vaccinated may be a consequence of the short duration of the IgM and IgA antibodies active at mucosal surfaces, especially the nasal mucosa. Both IgM and IgA antibodies vanish after 4-5 weeks post-induction. The longer-acting IgG antibodies are very likely key to reducing disease severity but are unlikely to prevent infection via the nasal mucosa or other mucosal surfaces.

Emerging SARS-CoV-2 variants throw another spanner in the works. New variants have shown to be more aggressively transmissible. Some studies have even indicated that certain variants evade immune-responses from naturally occurring or even vaccine antibodies. If this is the case, a vaccinated community may be ravaged by a variant. Such variants have the potential to infect and sicken even those who have been vaccinated. A recent study from Chongqing China shows that sera from those infected by the original Wuhan strain in January and February of last year have little to no ability to protect against either the UK (B.1.1.7) the South African (B.1.351) strains.

Ultimately, we need more data. As more countries continue to vaccinate in higher quantities, we will know important information like efficacy, transmissibility, infection among the vaccinated, and more. That information will roll in slowly but surely in the coming weeks.