health & science

Why fully vaxed hospital workers are getting measles

Counties Manukau DHB has confirmed that four of its front-line staff have had measles, including two students. Meanwhile immunisation experts have explained why some hospital staff are getting measles, despite being vaccinated.

Counties Manukau District Health Board has confirmed that four of its seven staff who caught measles this year were front-line health workers, including two healthcare students.

The other three were back-office staff.

An eighth worker - a DHB contractor - has also had measles during Auckland’s ongoing outbreak, says Counties Manukau.

However the DHB, which runs Middlemore Hospital, says there’s no reason to think any patients caught measles from its workers, because it has strict containment measures.

The seven staff at Counties Manukau are among at least 20 DHB workers to have caught the highly infectious virus around the country so far this year - a figure that excludes health contractors, GPs, midwives and others not directly employed by health boards.

Some hospital workers went to work while contagious, requiring DHBs to trace patients and fellow staff and ask any unvaccinated contacts to stay isolated for three weeks. 

Earlier this year, six staff caught measles on the job at Christchurch Hospital, a development that prompted a measles expert to tell an international conference in March that measles could rapidly get out of control once it was spreading in hospitals. 

Health authorities managed to contain the Christchurch outbreak, and avert a second outbreak in the city. However, Auckland’s measles crisis is still ongoing.

More than half of the almost 2000 confirmed infections nationally have shown up in one DHB area, Counties Manukau, which has had at least 62 new cases confirmed in the past two weeks.

Because measles is going around in Auckland, Counties Manukau DHB says it is difficult to know where its seven workers caught the virus.

The infected number includes two healthcare students, whom the DHB doesn’t have immunity records for. A spokesperson for the DHB said universities are responsible for the immunity screening of their students.

However, the DHB does screen its clinical staff - and says the two clinical staff who caught measles had previously showed immunity to the disease when tested as part of their pre-employment screening. (The back office staff at the DHB don’t appear to have been screened).

Why are fully vaccinated staff getting sick?

The pair of clinical workers are not the only health workers to have had measles in New Zealand this year despite being fully vaccinated, or having shown immunity to the virus.

Two out of the six infected Christchurch Hospital staff were fully vaccinated and a third was partly vaccinated, having had one of the two recommended doses of the vaccine.

At first glance, the figures seem oddly high, given the measles vaccine is highly effective for most people. 

Receiving two jabs over your lifetime gives 95-99 percent of people immunity to measles, says Nikki Turner, the director of the Immunisation Advisory Centre. Only a tiny percentage of people won’t develop immunity after having both doses, and a further small proportion will develop some immunity - but only enough to reduce the severity, not completely avoid measles. 

On closer inspection, however, it turns out the infection figures among staff who are known to be immunised at DHBs are about what you’d expect from a DHB with high, though not necessarily full, measles vaccination rates.

While most healthcare staff will have been vaccinated against measles, if a large number of staff - say, more than 100 - were exposed to the super-contagious virus, a few of the vaccinated people will still catch it (though they may get a milder dose). Meanwhile, virtually every unvaccinated staff member who's exposed to measles will catch it. The end result may be that about half the measles cases are in fully vaccinated people, even though, individually, their chance of getting sick was a tiny fraction of those who weren't unvaccinated.

For example, let's say a fictional hospital has 100 measles-vaccinated staff for every two unvaccinated staff. (Based on data supplied to Newsroom, that's a plausible vaccination rate among staff in some high-risk wards in New Zealand hospitals.)

If all 102 staff get exposed to measles, both of the unvaccinated staff would be very likely (90 percent likely, by some estimates) to catch it.

On top of that, one or two of the 100 vaccinated staff may also catch the virus, because the vaccine isn’t perfect. Either they haven't become immune after having two MMR does, or they might have become immune but the protection wasn't total, meaning they get milder measles.

The fact that they got sick doesn’t mean the vaccine isn’t working: being vaccinated has successfully prevented 98 percent of the staff who tried to get protected from getting measles, from getting it, meaning the chance of each individual immunised staff member catching measles was tiny. That's in contrast to the unvaccinated staff, who are almost certain to catch measles if they encounter it.

But because the hospital employs many, many more vaccinated people than unvaccinated people, the end result is that there are roughly equal numbers of vaccinated and unvaccinated staff with measles - in this example, two staff from each category.

If the fictional hospital had had full vaccination coverage, the number of staff with measles might have been reduced by half, to two people.

Protecting the herd

Ironically, according to Nikki Turner, the more people who are vaccinated in a community, the bigger the proportion of measles cases fully vaccinated people will account for.

But, overall, the number of people who catch the virus in the community will shrink dramatically.

Those who do catch it may get less sick.

Turner told Newsroom that immunised people were less likely to end up in hospital if they got measles, because of cases where the vaccine gives them some immunity, but not enough to totally prevent measles. “You can have some immune protection but it may not be totally protective - then you can still catch measles but it is likely to be more mild,” says Turner. “There are much higher notification rates of measles than (people who are) hospitalised in partially or fully vaccinated people, i.e. vaccination modifies the disease but doesn’t totally prevent it in some cases,” she says.

Ideally, in communities where almost everyone is vaccinated, measles notifications shrink to very low rates - or zero - because the virus is no longer passed around when it comes in from outside the community. 

New Zealand was verified by the World Health Organisation as having eliminated measles from its population in late 2017 after three years with no local transmissions. However, Turner says it was always foreseeable that our achievement would be short-lived, because of the large, well-known gaps in our immunisation coverage.

In 1991, less than 60 percent of two-year-olds were fully immunised, and the rates were even lower for Māori (42 percent) and Pacific (45 percent) toddlers. While the rates improved in following years, the bad years left immunity gaps that haven't been filled among today's young adults. One study found people born from 1980 to 1999 had about 83.4 to 84.6 percent immunity to measles - lower than what's needed to protect the population. 

For New Zealand to regain its herd immunity, there needs to be a catch-up programme for young and middle-aged adults who never got vaccinated, Turner wrote in the New Zealand Medical Journal this month. 

She noted closing the gap was not going to be easy. Not only is there a measles vaccine shortage, young adults tend to have "other life priorities", she wrote. "Calls for action can create over-vaccination in ‘worried well’ and still miss the unvaccinated."

As the number of confirmed cases keeps climbing, new research has described how the virus causes immune amnesia - inflicting lasting harm on people's immune systems.

The weakened immunity leaves people vulnerable to other dangerous infections such as flu and pneumonia for several years after having measles, which may explain why death rates among children from other illness also fall in countries whose measles vaccination rates increase.

Measles itself can cause serious complications. Transmission inside hospitals is considered especially dangerous, because of the number of patients who have low immunity and other vulnerabilities, including un-immunised new babies.

Hospitals have strict containment measures to prevent the spread of known cases, and people with suspected measles are told to wait in the car or at home for an assessment.

A Newsroom inquiry involving official information requests to every DHB tried to gauge the immunity levels of staff in New Zealand hospitals, including those working in high-risk wards such as maternity and ICU. While the Ministry of Health recommends a raft of vaccines, it isn't compulsory to have them, even for people working in areas such as newborn intensive care. 

Only two DHBs had figures on what proportion of their staff were immune to measles or other contagious, vaccine-preventable illnesses, such as whooping cough. The percentages ranged from no unvaccinated staff in one oncology ward, to 11 percent unvaccinated across a whole DHB, as of two years ago. Other contagious illnesses such as whooping cough, flu and chicken pox had much lower vaccination rates than against measles.

DHBs say their managers assess and deal with any risk from staff who decline to be vaccinated. The Ministry of Health says it is DHBs' responsibility not to roster unvaccinated staff to work with the most vulnerable patients.

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