How do covid vaccines protect you? Six questions and their answers

"Las vacunas nos ponen en contacto con una parte del virus porque producimos una respuesta inmunitaria que genera memoria", explica Eva Martínez, jefe del servicio de inmunología del Hospital Germans Trias de Badalona y vicepresidenta de la Sociedad Catalana de Inmunología. A partir de esta respuesta, "la segunda vez que se entra en contacto con el virus [debido a una infección natural], se genera una respuesta más rápida e intensa que lo neutraliza".

"Current vaccines offer good protection against infection, although it is never 100%", says Julián Blanco, head of virology and cellular immunity at IrsiCaixa. But what does it mean that protection is not 100%? "The probability of the virus reaching the lung in someone who has been vaccinated and causing serious disease is practically zero", he explains. "The available scientific data indicate that vaccines have a very high efficacy in preventing the disease, that is, covid-19 with symptoms, severe cases and hospitalisations, and also deaths", explains Magda Campins, head of the preventive medicine and epidemiology service at Vall d'Hebron Hospital. "The idea behind the vaccines is that if you do get the disease, you don't get it in a severe form", adds Martínez.

"There is still not enough scientific information available on whether vaccines prevent asymptomatic infection", says Campins. "If they did, then transmission would be prevented", she says. However, "data from Israel indicate that vaccines prevent asymptomatic infection in more than 80% of cases, and those from Scotland also point in the same direction", adds Dr. Campins. "However, these data are preliminary and we cannot yet state this categorically", she warns.

"When a vaccinated person comes into contact with the virus, it is most likely that the antibodies neutralise it", explains Martínez. "If this is not the case, the body mounts a very rapid immune response", she adds. This means that the period of time the virus is active in the body is reduced and, "the time during which you can become infected is shorter, and so is the amount of virus that can be transmitted". "The probability of contagion is small, but it exists", says Martínez.

"There are studies that indicate that vaccinated people with asymptomatic infection have a much lower viral load and, therefore, this would contribute to a reduction in transmission", says Campins. "It is likely that in vaccinated people the amount of virus that replicates is so small that it is effectively not detected or transmitted", confirms Blanco.

"All current vaccines protect against the British variant", says the IrsiCaixa researcher. "The Brazilian and South African variants are somewhat more resistant to antibodies, but most vaccines generate enough to block them", he adds. "The AstraZeneca and Janssen vaccines have lower efficacy against these variants, but they protect against severe disease and death, so the advantages of vaccination are very clear.

"The immune response to natural infection depends on the amount of virus it has been in contact with, whereas the response generated by vaccines is more homogeneous", explains Blanco. "The immune system responds according to the stimulus it receives", says Martínez: "if you tickle it, it responds with tickles, and if you kick it, it kicks back", which is why "people who have a mild illness generate fewer antibodies than those who have a severe illness". In general, according to Blanco, "vaccines generate more antibodies than natural infection". "In the hospital we are seeing very high levels of antibodies in vaccinated people", Martínez corroborates. For this reason, the IrsiCaixa researcher is of the opinion that "people who have had the disease should also be vaccinated, even if it is only with one dose".

Despite these differences, "natural infection protects against reinfection, which, if it occurs, will be milder than the first time and in most cases asymptomatic", says Campins. "Even if the first natural infection has been mild, the immune system has already been able to control the virus and, therefore, in case of reinfection there will be a faster and more effective response that will make it milder", adds Blanco.

"According to the studies that have been done so far, protection from natural infection lasts a minimum of seven months, while that from vaccines lasts a minimum of six months", Campins explains. This does not mean that this protection does not last longer, but that it has not yet been possible to carry out the studies to prove it.