LONDON — People who are fully vaccinated against Covid-19 are highly protected against severe infection, hospitalization and death caused by the virus. But coronavirus cases among the fully vaccinated — so-called “breakthrough” Covid cases — are still being seen among those who have had two doses.

While it’s rare for vaccinated people in the U.S. or Europe to get sick from Covid, breakthrough cases are happening for a number of reasons, experts note.

For a start, none of the vaccines being deployed in the U.S. or Europe are 100% effective at preventing infection.

In addition, new Covid strains such as the highly infectious delta variant — which is now prevalent around the world — have complicated the efficacy picture. There is also incomplete data into how long immunity from Covid lasts following vaccination.

The alarm was raised over breakthrough Covid cases when preliminary data in Israel — which had one of the fastest vaccination programs in the world — published in July found that the Pfizer-BioNTech Covid-19 vaccine was just 40.5% effective, on average, at preventing symptomatic disease.

The analysis, which was carried out as the delta variant became the country’s dominant strain, still found that having two doses of the shot provided strong protection against severe illness and hospitalization, however, the country’s Health Ministry reported.

The data also appeared to show a waning effectiveness of the Pfizer-BioNTech shot, however, with the vaccine only 16% effective against symptomatic infection for those individuals who had two doses of the shot back in January. But for people that had received two doses by April, the efficacy rate (against symptomatic infection) stood at 79%.

The Israeli data stands in contrast to a study in England carried out from April to May that found that, after two doses, the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease caused by the delta variant.

Comparing the results is tricky, however, given differences in the nature of the vaccination programs in both countries (Israel gave all its adult population the Pfizer vaccine, for example, while in the U.K. there are several vaccines in use, with the Pfizer-BioNTech shot predominantly given to younger people) as well as differences in the study dates, age groups and Covid testing regimes involved.

Similarly to the Israeli data, the English data also concluded that after two doses the Pfizer-BioNTech Covid vaccine is highly effective against hospitalization from the delta variant (the English data found the vaccine to be 96% effective, the Israeli data found it to be 88% effective against hospitalization, on average). In addition, English data found that the Oxford-AstraZeneca vaccine was 92% effective in preventing hospitalization after two doses.

Initial vaccine efficacy data following clinical trials, released by Pfizer and BioNTech last year, showed that the vaccine was 95% effective against infection from strains of the virus that were circulating at the time.

Read more: Moderna may be superior to Pfizer against delta variant — breakthrough odds rise with time

Professor Lawrence Young, a virologist at the University of Warwick’s Medical School in the U.K., told CNBC that cases of Covid in fully vaccinated people are a reminder that “no vaccine is 100% effective.”

“There will always be a proportion of individuals who will still remain susceptible to infection and illness,” he said Monday.

“There are also two other factors that impact vaccine effectiveness: (1) waning immunity — we still don’t know how long vaccine-induced protective immunity lasts. This is very likely to be a factor in those elderly and more vulnerable individuals who were vaccinated early in the vaccine rollout program,” he noted.

The second factor, he added, related to “breakthrough infections in vaccinated individuals due to the more infectious delta variant” which added weight to the case for booster vaccination programs, he said. As yet, the jury is still out on booster programs with a decision yet to be made in the U.S. and U.K.

It’s difficult to know the full extent of “breakthrough” Covid cases because cases in vaccinated people tend to be mild or asymptomatic and could go easily go unnoticed, but figures collected by NBC News has found that at least 125,000 fully vaccinated Americans have tested positive for Covid and 1,400 of those have died. Still, the 125,682 “breakthrough” cases in 38 states found by NBC News represented less than 0.08% of the 164.2 million-plus people (and counting) who have been fully vaccinated since the start of the year, or about one in every 1,300.

That is, the number of cases and deaths among the vaccinated is very small compared to the number among the unvaccinated. Health officials, particularly in the States, are urging unvaccinated people to come forward for Covid immunization.

For its part, the U.S. Centers for Disease Control and Prevention says that breakthrough cases are “expected” and that “there will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from Covid-19.”

As of May 1, the CDC said it had “transitioned from monitoring all reported vaccine breakthrough cases to focus on identifying and investigating only hospitalized or fatal cases.” It said the shift would help maximize the quality of the data collected on cases of greatest clinical and public health importance.

Andrew Freedman, a reader in infectious diseases at the U.K.’s Cardiff Medical School, told CNBC that “breakthrough” cases were to be expected.

“The vaccines are very good at protecting against severe infection, hospitalization and death but they’re les effective at protecting completely against infection and we know that many people who have been fully vaccinated are still getting delta infections with, in most cases, mild symptoms,” he told CNBC’s “Squawk Box Europe” on Monday.

“What we don’t know is whether giving an additional booster will actually increase protection and reduce delta variant infections,” he noted.

It must be emphasized that studies show that fully vaccinated people are much less likely to suffer from a Covid infection — or to contract the virus in the first place.

New research from the U.K. released last Friday showed that double vaccinated people were three times less likely than unvaccinated people to test positive for the coronavirus.

Analyses of PCR test results in the REACT-1 study — a major coronavirus monitoring program in Britain led by Imperial College London — also suggested that fully vaccinated people may also be less likely than unvaccinated people to pass the virus on to others, due to having a smaller viral load on average and therefore likely shedding less virus.

Professor Paul Elliott, director of the REACT program from Imperial’s School of Public Health, said the findings highlighted both the advantages and limitations of Covid vaccines.

“These findings confirm our previous data showing that both doses of a vaccine offer good protection against getting infected. However we can also see that there is still a risk of infection, as no vaccine is 100% effective, and we know that some double vaccinated people can still become ill from the virus,” he said.

Steven Riley, a professor of infectious disease dynamics at Imperial, said that so-called “breakthrough infections” in fully vaccinated people needed to be studied further, particularly as parts of the world contend with the spread of the delta variant.

“The delta variant is known to be highly infectious, and as a result we can see from our data and others’ that breakthrough infections are happening in fully vaccinated people. We need to better understand how infectious fully vaccinated people who become infected are, as this will help to better predict the situation in the coming months, and our findings are contributing to a more comprehensive picture of this.”