At the time of writing (February 2021), it is 1 y since the coronavirus disease 2019
(COVID-19) pandemic began to become a pandemic. To date, there have been >100 million
cases of COVID-19 diagnosed globally, leading to at least 2.2 million deaths.
1
The indirect effects of the pandemic on other health conditions, as well as economic
and societal well-being, are accruing at an alarming rate.
2
Probably due to its young population, perhaps combined with recent experience of combating
large-scale epidemics, such as Ebola in 2014 and 2016, sub-Saharan Africa has been
affected relatively modestly by COVID-19, with a death toll of about 60 000 cases
so far, mainly in South Africa. However, these reported numbers may be underestimates,
given the limitations of the local health systems. Also, the future course of the
pandemic is highly uncertain, especially where large-scale vaccination campaigns may
be more challenging than in high-income countries. Model predictions suggest that
suspension of control efforts for malaria, TB and HIV in low- and middle-income countries
could lead to deaths on a similar scale to those from COVID-19 itself.
3
Similar effects were seen during the Ebola outbreak in West Africa, where the number
of excess deaths due to other factors (including maternal mortality and particularly
malaria) was estimated to be of the same order of magnitude as the number of deaths
directly due to Ebola.
4
COVID-19 and neglected tropical diseases
Only recently, on 28 January 2021, the WHO celebrated the global achievements in striving
towards the 2020 goals for neglected tropical diseases (NTDs) and announced the roadmap
to achieve further goals in 2030.
5
This celebration of achievements combined with a future commitment to lessen the toll
of these diseases were overshadowed by the potential impact of COVID-19 on health
systems in populations where NTDs are endemic. In April 2020, the WHO issued a recommendation
to postpone mass drug administration (MDA) and other control activities for NTDs until
further notice, in recognition of the potential risks of these activities to increase
COVID-19 infection.
6
In July 2020, the WHO provided a decision-making framework, including a risk-benefit
assessment, for resuming or maintaining activities in the context of COVID-19 (guidance
on the risk analysis).
7
Many NTD-related activities have ceased and the picture of programme interruptions
is still uncertain.
As part of broad stakeholder engagement,
8
the NTD Modelling Consortium was asked by national programmes, donors, policymakers
and implementation partners to estimate the consequences of interrupting NTD interventions
and the impacts of mitigation strategies, especially regarding attempts to achieve
the 2030 roadmap targets, building on previous work in this area.
9
Initial results were presented on seven NTDs, namely, the Gambiense form of human
African trypanosomiasis (gHAT) and visceral leishmaniasis in the Indian sub-continent,
which are both controlled by case finding, as well as lymphatic filariasis, onchocerciasis,
schistosomiasis, soil-transmitted helminthiases and trachoma, which are controlled
by preventive chemotherapy through MDA.
7
–
11
The results were also presented at WHO webinars (https://www.who.int/news/item/26-05-2020-neglected-tropical-diseases-and-covid-19-who-holds-consultative-meeting-to-assess-impact-on-programme-implementation,
https://apps.who.int/iris/bitsteam/handle/10665/339238/sea-cd-328-eng.pdf?sequence=1)
and an extensive slide-deck was circulated to partners.
12
This special issue of Transactions of the Royal Society of Tropical Medicine and Hygiene
reports the more detailed findings of the modelling studies for each of the seven
NTDs following further stakeholder engagement.
During any interruption to a NTD programme, there is likely to be a resurgence in
infection and/or disease to levels above those which would have been observed if the
programme had been maintained. The extent and consequences of this additional transmission
depends on the dynamics of each individual NTD. For the seven NTDs studied in detail
here, we outline some of the potential drivers of high impact.
Epidemic growth rate
The dynamics of these seven NTDs are quite different and this has important consequences
for the resurgence of infection. Onchocerciasis
13
and lymphatic filariasis
14
generally have very slow epidemic growth rates, and so the resurgence of infection
during one missed round of annual MDA is generally very slight. Basically, the delay
in reaching the target is similar to the number of years that treatment rounds are
missed. The dynamics of gHAT
15
are also slow, meaning that short-term interruptions to case finding may have limited
impact. By contrast, visceral leishmaniasis in the Indian subcontinent,
16
as well as trachoma
17
,
18
and schistosomiasis,
19
have somewhat faster epidemic growth rates, particularly in high transmission areas.
For visceral leishmaniasis in the Indian subcontinent, as well as trachoma and schistosomiasis,
interruptions to programmes will therefore undermine the gains made over many years
and considerably delay achievement of the 2030 targets. For soil-transmitted helminths
20
there are a range of resurgence rates, approximately in proportion to the inverse
of the worm’s lifespan.
21
Accrual of morbidity
Although most of the models in this collection do not explicitly consider morbidity,
it is clear that the rate at which morbidity accrues for a given increase in prevalence
is also an important consideration in prioritising the restarting of programmes. For
schistosomiasis, visceral leishmaniasis and trachoma, particularly in high transmission
settings, the impact on morbidity or even mortality may be substantial.
16–19
Prevalence prior to interventions
For any NTD, the areas at highest risk of resurgence during interruptions to programmes
are those with high transmission rates, or high prevalence before interventions were
introduced. As resurgence is more rapid, the longer the delay then the longer the
time it will take to get back on track (e.g. a larger number of MDA rounds). The analyses
in this collection also highlight that the impact of an interruption early or late
in a programme may have different effects. For schistosomiasis and onchocerciasis,
the model predictions clearly show that programmes with shorter MDA histories will
be most vulnerable to interruptions, as the underlying transmission potential is highest.
13
,
19
The same can be seen for visceral leishmaniasis in the Indian subcontinent in terms
of the impact on cumulative visceral leishmaniasis incidence. However, if the impact
is viewed in terms of the delay in reaching the 2030 target (i.e. incidence below
1 visceral leishmaniasis case per 10 000 people per year), then the impact is highest
later in the programme (5–8 y), because the already achieved target will be lost again.
16
Extended delays to programmes and affected additional interventions
The longer the interruption to programmes then the greater the potential negative
impact. The analyses presented in this collection demonstrate that these are non-linear
effects, especially for NTDs with a high epidemic growth rate, and the second year
may have a higher impact than the first year of delay depending on the epidemic growth
rate and the prevalence at the time of interruption. Similarly, for NTDs that benefit
from additional interventions, such as vector control (for onchocerciasis) or bednets
(for lymphatic filariasis), interruption of these interventions will further increase
the overall impact.
13
,
14
Coverage and efficacy when programmes return
COVID-19 has had diverse impacts on programmes and some commentators suggest
2
,
22–24
that there may be issues regarding coverage for MDAs when programmes are implemented
in 2021 and beyond. For most of the simulations in this collection, the results contain
an assumption that programmes will return with the same coverage and efficacy as was
in place before 2020. If this is not achieved then mitigation of the impact of COVID-19
on NTD programmes will be extremely challenging.
Mitigation strategies
Ideally, interruptions of NTD control due to the COVID-19 pandemic should be minimised
and programmes should start again as soon as possible. Mitigation strategies will
help all NTD programmes to recover the lost ground, but in different ways. In the
analyses within this issue, additional or enhanced MDA programmes were simulated for
the five diseases for which MDA is the major intervention. For yearly MDA programmes,
this included providing the 2020 round in 2021, switching to biannual treatment from
2021, including adults in school-based programmes (for soil-transmitted helminthiases
and schistosomiasis) or only targeting children (for trachoma). These mitigation strategies
have important logistical and economic costs, and the expected coverage in these alternative
scenarios should be carefully considered. However, their inclusion in these analyses
provides an epidemiological investigation of their potential to mitigate the impact
of COVID-19-related delays.
Clearly, increasing the number or coverage of MDAs reduces the level of infection
more rapidly, but there were some interesting results. For onchocerciasis, for example,
biannual MDA turned out to be a better choice for mitigation than increasing coverage.
13
For soil-transmitted helminthiases, mitigation may not be strictly necessary to achieve
the 2030 targets, as the programmes will catch up before this date. However, the duration
until catching up will be shortened, thereby reducing the impact on morbidity. For
schistosomiasis, increasing coverage and extending the programmes by treating adults
will both help to achieve the targets. In particular, for schistosomiasis and trachoma,
the authors propose that maintaining mitigation strategies will be the only way to
achieve 2030 targets in high-endemicity settings, as the recommended strategy would
not have managed this.
25
,
26
In this way, the possible mitigation strategies would not only allow programmes to
catch up to where they would have been, but also to accelerate achieving the 2030
goals.
For the case-finding disease (in this special issue that includes visceral leishmaniasis
and sleeping sickness [or gHAT]), interruptions to either passive or active case detection
is likely to lead to a build-up of undetected cases that will need to be addressed
when programmes return to full strength, and these cases may take some time to be
addressed and for transmission to reduce again.
15
,
16
Modelling and policy
The studies in this special issue have benefitted from the models that have been developed
by the NTD Modelling Consortium over the 6 y of its existence; some of them even have
a history of many more years to decades.
27
For gHAT, onchocerciasis and trachoma, the calculations were based on two (or for
lymphatic filariasis, three) different models per NTD capturing the same basic mechanisms
of transmission, natural history and control, but using different underlying assumptions
and methodologies. The fact that multiple models largely agreed regarding the predicted
impact of interruptions and mitigation strategies supports the general outcome of
the different papers.
The studies further benefitted from the partnerships and discussions that underlie
the modelling decisions on the right scenarios and outputs to study. The global NTD
community has supported this work through both the WHO and the Expanded Special Project
for Elimination of NTDs (ESPEN),
8
as well as by national programme managers engaging with the early publication of the
report. This is the first special issue produced by the NTD Modelling Consortium that
explicitly accounts for the extent to which its five principles for policy-relevant
modelling have been followed.
28
This was achieved by providing the policy-relevant items for reporting models in epidemiology
of neglected tropical diseases (PRIME-NTD) Table in the supplements of each of the
papers. The PRIME-NTD Table contains a checklist of how each principle was satisfied
and where this can be found in the manuscript. The first and perhaps most important
principle (i.e. engaging stakeholders throughout) was only met to a limited extent,
given the short time span in which to deliver the outcomes of these studies. However,
mainly existing models were used, and most of them, especially those with a long history,
have had involvement of various stakeholders at different stages of model development
and extension. The next three principles (i.e. a complete description of models, the
data used and degree of uncertainty) are basic components of good modelling practice
and are dealt with by detailed supplements, including uncertainty and sensitivity
analyses. The fifth and final principle (i.e. to provide testable model outcomes)
is extremely well satisfied by the many presented trends of how the number of infections
may change as a consequence of COVID-19-induced interruptions of control and the chosen
mitigation strategies. In the coming years these trends can be compared with data
to help in validating and further improving the models.
Looking forword
Over the last year there has been broad discussion that this may be an opportunity
for innovation in response to the crisis, and this collection contributes to this
discussion by highlighting that practical steps are needed to deliver this potential,
29
that novel methods for data collection can optimise the information underlying decisions
30
and, importantly, that novel strategies can deliver not just mitigation but acceleration
of programmes.
31
Importantly, as the modelling analyses in this special issue demonstrate, NTD programmes
need to be functioning at the same, or at an even higher level than previously, to
mitigate the impact of this devastating pandemic on NTDs.